In Tamriel, a birthsign is the constellation under which a person is born. Each bestows upon those born under it unique magical powers, either passive or active, and may be perceived as either a "blessing" or a "curse". The thirteen birthsigns of Tamriel are:


General Tips
The birthsigns can be categorized into three general groups: those that give Attribute bonuses, those that provide lesser and/or greater powers, and those that affect your Magicka:

Attribute bonuses: Lady, Steed, Thief, and Warrior.
Magicka effects: Apprentice, Atronach, and Mage
Greater/lesser powers: Lord, Lover, Ritual, Serpent, Shadow, and Tower
All Attributes are capped at 100. The birthsigns Warrior, Thief, Lady, and Steed won't allow you to go above this cap.

A character's chosen birthsign cannot be changed in-game (without the use of the console), but Doomstones can provide additional powers -- sometimes, even permanent ones -- that are similar to, and in some cases better than, those of the birthsigns, such as the Shadow (become invisible for 60 seconds) vs the Jone Stone (become invisible for 120 seconds).


Apprentice

The Apprentice is a birthsign which affects your Magicka, bestowing the Apprentice Birthsign ability (FormID 00022A3E), which has two permanent effects:

Fortify Magicka 100 points on Self
Weakness to Magic 100% on Self
In-Game Description: The Apprentice ability confers a 100-point bonus to your Magicka attribute, but gives you a 100% Weakness to Magic.
Load Screen Description: Those born under the sign of The Apprentice have increased Magicka, but also have a weakness to it.
Emperor's Commentary: "Today the Apprentice shall serve you at the forge of destiny."

Tips
Conjuration is very valuable in providing cover, especially by taking blows from magic for you. An Apprentice caster is going to be fragile against all forms of magical attack, so learn to love your pets.
Remember that a High Elf already has a weakness to fire, frost, and shock magic. Altmer weaknesses + 100% Weakness to Magic = quick death against a spell caster, however, if you can find a way around these weaknesses, say, a resist magic, or absorb magic spell, this can be a very good combination.
A Breton's natural Resistance to Magic can effectively halve the Apprentice's disadvantage and add 100 Magicka to their already substantial Magicka reserves.
One distinct advantage of this birthsign is that it gives you access to the Weakness to Magic effect for spell making and enchanting. Custom Weakness to Magic spells can be used to devastating effect, especially if you cast them one after the other. The only buyable Weakness to Magic spell in the game requires Journeyman level in Destruction, one of the more difficult magic skills to raise.
There are a number of items with Resist Magic, Reflect Spell and Spell Absorption which you can find to help protect you. An apprentice will find three of the Daedric artifacts particularly useful: The Ring of Namira, Saviour's Hide, and Spell Breaker.
When you get access to the Arcane University, customize all self-target spells with a marginal ailment (i.e., Drain Fatigue 1pt). This will render the spell "harmful", thus causing it to be magnified 100% by your Weakness to Magic. This is a wonderful way to turn your glass cannon "Weakness" into a unique and fun advantage, effectively doubling the power of healing, protection, and fortify spells on yourself.
This exploit does not work if you have installed the version 1.1.511 patch.
Any active Spell Absorption effects can ruin these custom spells.

Atronach

The Atronach is a birthsign which affects your Magicka, bestowing the Atronach Birthsign ability (FormID 00022A3D), which has three permanent effects:

Fortify Magicka 150 points on Self
Spell Absorption 50% on Self
Stunted Magicka on Self (prevents standard magicka regeneration)
In-Game Description: With the Atronach ability you don't regain Magicka over time. Instead you have a 50% Spell Absorption to recharge your Magicka. Your base Magicka is also increased by 150 points.
Load Screen Description: Those born under the sign of The Atronach cannot regenerate Magicka, but have a chance of absorbing any magic cast at them.
Emperor's Commentary: "Today the Atronach shall aid you with your appointed burden."

Tips
This birthsign is an important component for maximizing your Magicka.
To overcome Stunted Magicka and effectively regenerate magicka, you will want to take full advantage of various techniques and tips. See:
Restore Magicka: potions, Welkynd Stones, Ayleid Wells, etc.
Spell Absorption: get your Spell Absorption to 100% if possible; absorb all possible spells.
Absorb Magicka: enchant weapons or (at high Restoration levels), cast spells.
Even if you never intend for your character to cast magic, you can still benefit from the Atronach's Spell Absorption. As a warrior, for example, you would simply absorb half of the spells cast at you, making combat much easier against spellcasting opponents. If you can increase your Spell Absorption to 100%, you have a character who is immune to magic.

Lady

The Lady is a birthsign which affects your Attributes, bestowing the Lady's Blessing ability (FormID 00022A40), which has two permanent effects:

Fortify Willpower 10 points on Self
Fortify Endurance 10 points on Self
In-Game Description: The Lady's Blessing confers bonuses of 10 points to your Willpower and Endurance attributes
Load Screen Description: Those born under the sign of The Lady have increased Willpower and Endurance.
Emperor's Commentary: "Today the Lady shall fortify you in your quest for glory."

Tips
The total Health points you gain each level is determined by your Endurance at the time you level. Thus, increasing your Endurance as quickly as possible early in your character's life will give you the highest potential Health later on. This early 10-point boost can be a big help later on.
If you don't want to play the Atronach birth sign but are still planning on a High Elf (Altmer) spell caster, this is another good choice (along with The Mage Birthsign). It boosts your weak Willpower and Endurance to 50 each (if you are a male, but is still useful for the more frail female Altmers).
Nords, Argonians, Dark Elves, and Wood Elves may also find this birth sign useful.
Not recommended for Orcs, who start with high Endurance and Willpower.

Lord

The Lord is a birthsign which provides a lesser power, balanced by one permanent weakness:

Blood of the North lesser power (FormID 00022A41): Restore Health 6 points for 15 seconds (90 points total) on Self, multiple times per day at a cost of 50 Magicka
Trollkin ability (FormID 00022A42): Weakness to Fire 25% on Self, permanent
In-Game Description: Being born under the Lord gives you the Blood of the North lesser power to regenerate up to 90 points of Health. However, you also gain the Trollkin curse, a permanent 25% Weakness to Fire.
Load Screen Description: Those born under the sign of The Lord can regenerate Health, but are vulnerable to fire.
Emperor's Commentary: "Today the Lord shall prove a faithful patron as you confront your fate."

Tips
Blood of the North allows the player to cast a Restore Health effect at magnitude 6 for 15 seconds (90 points of healing) for an incredible 50 Magicka . This is by far the most Magicka-efficient healing spell in the early and middle game, until you achieve a high Restoration skill. The next most efficient healing spell is the one that all characters get at the beginning of the game, Heal Minor Wounds, which is far less effective. A Lord gets an expert level, Magicka -light healing spell right from the start of the game.
This sign is especially useful for combat based characters, as the spell may be cast multiple times to top up health levels in the middle of a fight, and it would take a long time to gain a high enough Restoration skill to cast an equally effective healing spell.
This healing ability is also a Lesser Power, so it is unaffected by any armor your character may be wearing.
Conversely, this sign is not recommended for magic based characters, especially if Restoration is a major skill, because you do not gain experience by using Lesser Powers.
The 25% weakness to fire can be countered by wearing a piece of armor with a Resist Fire enchantment. You could either custom-make this, or find it in random loot. Dunmer characters have a natural resistance to fire, so are particularly suited to this sign. Altmer, on the other hand, will probably want to avoid this sign, due to their innate weakness to fire.

Lover

The Lover is a birthsign which provides a greater power:

Lover's Kiss greater power (FormID 00022A43): Paralyze 10 seconds on Touch and Damage Fatigue 120 points on Self, once per day
In-Game Description: Use the Lover's Kiss power once a day to Paralyze an opponent for 10 seconds at the cost of 120 points of Fatigue.
Load Screen Description: Those born under the sign of The Lover can paralyze others with a kiss.
Emperor's Commentary: "Today the Lover shall sweeten your journey as you confront your fate."

Tips
The Serpent Birthsign Doomstone grants the Cobra's Dance greater power, which provides Paralyze for 5 seconds, accompanied by Damage Health 4 points for 20 seconds on target (80 points total). There is no Fatigue penalty associated with Cobra's Dance. Although shorter in duration, some players may prefer to use the Doomstone effect to obtain a paralysis greater power.
The Lover's Kiss power grants the Paralyze magical effect at spellmaking altars if you are having difficulty finding the spell. Be warned though, any Paralyze spell has a very high Magicka casting cost and you have to be at least an apprentice illusionist in order to cast the weakest (1-2 second duration) custom spells.
Your character might faint when this spell is cast if your fatigue is already low from jumping or swinging weapons, so you might still be on the floor when your angry victim gets back up and takes a vengeance on you.
Imperials can use their Star of the West greater power to absorb 100 points of Fatigue, mostly compensating for the damage caused by casting Lover's Kiss.

Mage

The Mage is a birthsign which affects your Magicka, bestowing the Mage Birthsign ability (FormID 00022A31), which has one permanent effect:

Fortify Magicka 50 points on Self
In-Game Description: The Mage ability confers a permanent bonus of 50 points to your Magicka.
Load Screen Description: Those born under the sign of The Mage have increased Magicka.
Emperor's Commentary: "Today the Mage shall light your way on the paths of glory."

Tips
This birthsign is well suited for mages who want a Magicka bonus without any side effects; the other birthsigns that provide Magicka bonuses (Apprentice and Atronach) both have negative side effects.

Ritual

The Ritual is a birthsign which provides two powers:

Blessed Word lesser power (FormID 00022A3C): Turn Undead up to level 25 (100 points) for 30 seconds on target, multiple times per day at a cost of 40 Magicka.
Mara's Gift greater power (Form ID 00022A3A): Restore Health 200 points on Self, once per day
In-Game Description: Those born under the Ritual use the Mara's Gift power once a day as a powerful Restore Health spell. The Blessed Word can turn the undead.
Load Screen Description: Those born under the sign of The Ritual can heal themselves and turn the undead.
Emperor's Commentary: "Today the Ritual shall speed you on your star-patterned path."

Tips
Blessed Word allows the player to craft additional Turn Undead spells at spell making altars without the need to have another known Turn Undead spell.
The restore health for 200 points is an instant effect much like a strong potion of healing (better in fact). This is quite useful in a pinch up to about level 15 (completely healing you in most cases). While its effects are less pronounced in at the highest levels, it maintains its usefulness throughout the game at any level.

Serpent

The Serpent is a birthsign which provides one greater power, Serpent Spell (FormID 00022A63), which can be cast once per day, simultaneously casting all four of the following effects:

Damage Health 3 points for 20 seconds (60 points total) on Touch
Cure Poison on Self
Dispel 90 points on Self
Damage Fatigue 100 points on Self
In-Game Description: Gain the Serpent spell to cause a slow but potent poison on touch, while simultaneously curing yourself and dispelling magic on yourself. Casting this spell costs 100 points of Fatigue.
Load Screen Description: Those born under the sign of The Serpent can poison others at a loss of their own Fatigue.
Emperor's Commentary: "Today the Serpent shall sting the foes who seek your blood."

Tips
The Serpent Birthsign Doomstone grants the Cobra's Dance greater power, which provides Damage Health 4 points for 20 seconds on target (80 points total), accompanied by Paralyze for 5 seconds. There is no Fatigue penalty associated with Cobra's Dance. Therefore, the doomstone is generally a more effective way to gain access to this ability.

Shadow

The Shadow is a birthsign which provides one power:

Moonshadow greater power (FormID 00022A66): Invisibility 60 seconds on Self, once per day. As with all invisibility effects, the power is immediately dispelled if the player interacts with any item (doors, containers, etc.)
In-Game Description: Use the Moonshadow power once a day to become Invisible for 60 seconds.
Load Screen Description: Those born under the sign of The Shadow can make themselves invisible.
Emperor's Commentary: "Today the Shadow shall hide you from destiny's cunning hounds."

Tips
Once your renown is 10 you can visit the Jone Stone, and acquire the ability to become invisible for 120 seconds, once per day. This is twice as long as the Moonshadow power granted by The Shadow, making this birthsign of limited value.
Like other birthsigns with spell-like powers, this birthsign allows the caster to craft lower-leveled Invisibility spells at spell crafting altars.
Very useful for thieves and assassins, although only being able to cast Moonshadow once per day can be a significant limitation.
In the Black Marsh, Argonians born under the sign of the Shadow are called "Shadowscales;" they are given to the Dark Brotherhood at birth and are trained to be assassins. This is part of the back story behind the quest The Renegade Shadowscale, but it does not otherwise affect game play. In particular, the player is not treated differently if the player chooses to play an Argonian with the Shadow birthsign.

Steed

The Steed is a birthsign which affects your Attributes, bestowing the Steed Birthsign ability (FormID 00022A6A), which has one permanent effect:

Fortify Speed 20 points on Self
In-Game Description: The Steed ability grants a bonus of 20 to your Speed attribute.
Load Screen Description: Those born under the sign of The Steed can move faster.
Emperor's Commentary: "Today the Steed shall speed your progress on the road to destiny."

Tips
While some would not consider this to be a good birthsign, the proof is in the numbers. A Wood Elf (Bosmer) with a class that has Speed as a focus (such as Thief or Rogue) would have a starting Speed of 75 when taking this birth sign. Early in the game, the Speed attribute is very easily increased. By level 6, such a character has the potential of a 100 Speed. This rivals the effects of using horses and simple walking actually becomes tolerable. Since Speed (and the Acrobatics skill) affects how high you can jump, this can give you access to places to snipe your enemy from safety. Fortifying Speed past 100 is even more beneficial for similar reasons. Races with low initial values for Speed, such as Orcs, may also find the 20-point boost to Speed useful in order to overcome the deficiency. Also if you hate the drag down of Heavy Armor, this could be a helpful birthsign for you as well.

Thief

The Thief is a birthsign which affects your Attributes, bestowing the Thief Birthsign ability (FormID 00022A6B), which has three permanent effects:

Fortify Agility 10 points on Self
Fortify Luck 10 points on Self
Fortify Speed 10 points on Self
In-Game Description: The Thief ability grants a 10-point bonus to your Agility, Speed, and Luck attributes.
Load Screen Description: Those born under the sign of The Thief have increased Agility, Speed, and Luck.
Emperor's Commentary: "Today the Thief shall guide your steps on the road to destiny."

Tips
Luck is the slowest of all the attributes to raise, since unlike the others, it is not associated with any skills. This 10-point boost means you could potentially have all your stats maximized 10 levels earlier than you otherwise could. And since Luck can only ever be increased by 1 point per level, this combined with a custom class that has Luck as a primary ability saves you 15 levels of luck boosts.
Because Luck affects all skills, the 10 point boost in luck is handy. It equates to a 4 point boost in all of your skills as far as effects (but not perks) are concerned. If you customize a class to have Luck as a class ability and combine that with this birthsign, you start the game with every skill having an effective boost of +6. See the Efficient Leveling +5/+5/+5 vs +5/+5/+1 information in the character building tips for more on this.

Tower

The Tower is a birthsign which provides two powers:

Tower Key greater power (FormID 00022A6C): Open Average Lock (40 points) on Target, once per day
Tower Warden greater power (Form ID 000CB623): Reflect Damage 5% for 120 seconds on Self, once per day
In-Game Description: With the Tower Key power, once a day open a door or container of Average lock level or less. The Tower Warden reflects five points of damage for 120 seconds once a day.
Load Screen Description: Those born under the sign of The Tower can unlock doors magically, and the Tower Warden power reflects damage from attackers.
Emperor's Commentary: "Today the Tower shall prove a stout refuge in time of need."

Tips
The Tower Birthsign Doomstone grants the Warden Key greater power, which provides Open Hard Lock in 15 feet on Target, once per day, and therefore provides a more powerful open spell. The Tower Stone also provides The Master's Hand greater power, with Fortify Armorer 20 points for 120 seconds on Self, once per day.
No Doom Stones provide any Reflect Damage effect. In fact, this birthsign is the only (Non-Shivering Isles) way for your character to access Reflect Damage at the Spellmaking Altar to make custom spells. However, you still cannot make custom enchantments with it.
This is not a great sign for any thief-like character as any amount of skill in security coupled with a high agility will make easy work of any average lock. Also, any good thief will want practice in security to help boost his agility modifiers.
After playing the lock-picking mini-game long enough, you can pick any lock in the game with one or two lock picks and a security skill of 5, rendering this ability moot. It's possible to quest for an artifact called the Skeleton Key - an unbreakable lock pick - that will allow any character to open any lock with a little patience.

Warrior

The Warrior is a birthsign which affects your Attributes, bestowing the Warrior Birthsign ability (FormID 00022A6E), which has two permanent effects:

Fortify Endurance 10 points on Self
Fortify Strength 10 points on Self
In-Game Description: The Warrior ability grants a bonus of 10 points to your Strength and Endurance attributes.
Load Screen Description: Those born under the sign of The Warrior have increased Strength and Endurance.
Emperor's Commentary: "Today the Warrior shall prove a stalwart companion when fortune fades."

Tips
The total Health points you gain each level is determined by your Endurance at the time you level. Thus, increasing your Endurance as quickly as possible early in your character's life will give you the highest potential Health later on. This early 10-point boost can be a big help later on.
Male Redguards that are planning on using a stealth-based character will find this sign of particular use because both Strength and Endurance will be difficult to raise. Starting both of these stats at 60 makes a huge difference in the survivability of such classes.