The AionSource.com Aion Wiki has broken down each of the main Archetypes into their subclasses and offered up descriptions on the function of each.

The Archetypes are what your character will start out as, and progress into the subclass based upon the decisions and skillsets you choose as you increase your level.
Each Archetype, and subclass for the Archetype, is listed below and will take you to the Wiki article attached:
Mage -
incredibly powerful and yet exceptionally vulnerable - able to deal out
terrifying, awe-inspiring levels of damage, while taking little in return.
- Sorcerer - follows
the path of magic, mastering the elements of water, fire, wind, and earth.
- Spiritmaster -
ability to summon and control fierce creatures based on the four cardinal
elements of Earth, Fire, Wind and Water.
Priest
- arguably the backbone of any successful party. Their ability to
keep Daeva battle-ready, cannot be underestimated.
- Cleric - provide
support to allies through healing or fortification, but they can also be
formidable soldiers in battle using a mace and shield.
- Chanter - A specialty
in healing and fortifying magic, but they can also use a magic staff to great
offensive effect.
Scout
- Scouts concern themselves more with speed, camouflage, agility and awareness.
- Ranger - Adept in use
of the bow, they are also masters of the sword and dagger and adept in using
traps.
- Assasin - Able to
approach the enemy stealthily, then deal high damage with one mighty blow,
targeting the enemy’s vital areas.
Warrior
- Fearsome and hulking, a Warrior’s way is found more through brute force and
great resilience than guile and trickery
- Gladiator - Skilled
in using a variety of weapons, ranging from swords to halberds.
- Templar - The Templar
provides shelter from enemy fire, absorbing much of the brunt in battle. This
class also has a specialty in protection chants.