The concept of Birthsigns, first introduced in Morrowind, continues in Oblivion. As part of the creation of your character, you choose one of thirteen astronomical constellations as your birthsign. Your birthsign provides certain advantages to your character (sometimes offset by disadvantages) and once you choose, you're stuck with it.

Sign Benefit Comments
Apprentice +100 Magicka
100% Weakness to Magicka
A dicey combo. The extra magicka is nice, but the weakness to magicka-based spells (like Paralyze) can make life difficult
Atronach +150 Magicka
50% Spell Absorption
no Magicka regeneration
A great sign for a Sorcerer class, decent for any spell-casting class, though the lack of Magicka regeneration might be a problem
Lady +10 Willpower
+10 Endurance
A nice bonus for a combat/magic character
Lord Restore Health (6 pts for 15 seconds)
25% Weakness to Fire
Nice for those without great Restoration skills
Lover Paralyze (10 seconds on touch, drain Fatigue 120 points) Useful at any level, particulaly useful for stealth characters in getting around some problem NPCs
Mage +50 Magicka Pretty much a standard choice for spellcasters, nice for non-spellcasters at later stages of the game when they may have enough magicka to sling a few of the bigger offensive spells
Ritual Restore Health (200 pts) 1/day
Turn Undead (100% for 30 seconds)
Useful for any class at any level
Serpent (Damage Health [3 pts for 20 seconds on touch], Dispel [90%], Cure Poison, Damage Fatigue [100 pts on self]) Pretty weak effects and the Fatigue damage might be problematic in a combat situation, so probably not very useful
Shadow Invisibility (60 seconds) 1/day A useful effect for stealth-based characters; it can be duplicated by the stealth/magic combos, but it might be good for a quick getaway when you're out of other options
Steed +20 Speed It's nice to be able to outrun the opposition, but the skills governed by Speed are not terribly critical for non-stealth characters
Thief +10 Agility, +10 Speed, +10 Luck An excellent choice for stealth-based characters
Tower Open (average) 1/day
Reflect Damage (5% for 120 seconds)
A handy sign for any class at almost any level, but spellcasters may want to take a pass since the effects can be cheaply duplicated
Warrior +10 Strength, +10 Endurance Perfect for combat-based classes, useful for any others that are going to rely on melee or have races with lowered stats