Either Join Group Chat, Ask Kinship or read here for info on the 6 creep classes:
Orc Defiler: Healer based class, uses mostly hots at low rank and has some AoE debuffs
Orc Reaver: Hard DW DPS and is the best Melee DPS class for Raw Damage
Uruk Blackarrow: Ranged DPS, pretty straightforward.
Uruk Warleader: Healer/Tank Hybrid that does more cast heals then hots and has buffs.
Spider Weaver: Ranged CC/Snare and some DPS
Warg Stalker: Stealth Takedown class and some decent Melee DPS