Sorcerer is the only real caster in the game. Once you get Fireblast (main AoE) and Mana Barrier (absorbs dmg) you can solo things very easily. Still pretty easy getting there just have to be a bit more careful.
Then there are the two supports, the Priest and Mystic. The Priest is your big healer and the Mystic may not have huge heals but they provide great support and probably the most preferred healer for skilled groups with their crit aura (increases crit rate by 100%)
Healing in this game is actually fun. You move around avoiding bosses while casting heals which you lock-on. While at lower levels you may be purely healing... in end-game dungeons you play crucial roles in parties such as providing MP support, buffs, and usually the key person in boss mechanics.
Priest is easy to solo level while the Mystic may want a leveling partner.
EDIT:
http://www.twitch.tv/resonancestream/c/1867166
This is our group doing the end-game instance, this was our first kill. It's from a Sorcerer POV and he's got his rotations down. He doesn't have his best disc yet but the damage is there.
There's the Lancer (me), Sorcerer, Slayer, Warrior and Priest in this one.