Think of it like this -
Fighters (Armsman, Reaver, Mercenary, Paladin) - primary job is absorb damage, thanks to heavier armor and higher hitpoints. Secondary job, damage dealing thanks to weaponskills and/or spells.
Healers (Cleric, Friar, Heretic) - primary job is keep people alive, thanks to healing spells and resurrect abilities. Secondary job is damage dealing, thanks to weapons/spells. This group also has access to buffs to make others (and self) better.
Mages (Cabalist, Sorcerer, Theurgist, Wizard, Necromancer) - primary job is to do damage, either with pet or self. Secondary job is to do debuff or crowd control, depending on class/spell selection.
Rogues (Infiltrator, Scout, Minstrel) - Primary job is stealth, thanks to being the only groups that can hide. Secondary job is damage or crowd control.
SOME specs cross the lines - heri-tank version of heretic, fighter-varient of Friar, and crit/poison spec rogue are just a few. Most important, is to find what kind of character you LIKE to play and find a class that is most like what you like to play. If you want to play alone all the time, you are going to want a class that can do that. If you are looking primarily into RvR, you are going to want to avoid certain classes. Best suggestion, find a guild or buddy with someone, if you find you dont like the class - start again.