Warrior: offensive support via might and banners, works as power or condi but DPS is low either way, good CC.

Guardian: good DPS, defensive support from blocks and protection, good CC.

Revenant: good DPS, pulses boons (easiest source of fury) and increases boon duration (makes chrono better), best CC.

Thief: highest single-target DPS on small hitboxes, second best CC but pretty much a pure DPS class that does that job very well.

Ranger: heal/support, almost exclusively seen as druid healer but can also condi, very strong group offensive buffs (grace of the land, frost/sun spirits).

Engineer: highest condi DPS but in practice is too hard to play (by far the most complex rotation) for a class that offers very little over necro.

Elementalist: highest DPS against large hitboxes/large groups of mobs. Again, DPS is pretty much all it does and it's good at it. DPS recently nerfed to bring it into line with thief and guard. Can also heal, but they nerfed that build somewhat.

Mesmer: pure support class, chrono is the only reliable source of quickness in the game, best projectile reflects, best pull skill to move mobs together, does essentially no damage on its own.

Necromancer: good condi DPS, good CC, good at clearing condis from teammates, two necros do more than twice the damage of one necro because of the skill Epidemic so they are typically used in pairs if they are used.