Starting off you are able to pick four very general classes in which allows you to embark on your human journey into ascension at level ten.
Templar - The templar is your all around tank. Being able to specialize in swords, maces and great-swords they usually wield a sword and shield. They also tend to hold hate better (that's rage for you WoW players), have better blocking abilities and have overall more hitpoints. All in all, this is your protection spec warrior.
Gladiator - This class is your arms or furry warrior. Being able to equip almost every wepon in game, you are a DPS machine. The ability to wear plate allows you to take copious amounts of damage and most often times be an excellent off tank.
Assassin - The assassin is your rouge class. With the abilities of stealth, poison and most moves being completed on the targets back, this class can deal out a large amount of damage. Unlike the gladiators though, they wear leather and can go down fast.
Ranger - Equipping a bow and arrow, this is the only ranged physical DPS class in Aion. The downside of this class is you get no pet and constantly have to kite mobs around. Yet, on the positive side, you get many slows, snares and debuffs that you can stack on targets making the Ranger one of the hardest classes to kill.
Spiritmaster - This is your warlock. Being able to summon an assortment of pets (fire, water, earth and water pets), they are one of the best soloing classes.
Sorcerer - A mage by any standard, they are adpet in all the elemental magics. Being able to push out massive amounts of damage in a short time, they are the best burst damage in game. Watch out though, they are squishy.
Chanter - Your basic paladin, this class has a large array of buffs and can DPS and tank pretty well while soloing.
Cleric - Clerics are the overall best healers in the game. Armed with heals over time, instant heals, and various buffs, they are a necessity in every group or PvP battle. Basically your holy priest, they have little DPS capabilities until higher level