In coh/cov, you are a hero or villian with powers. You level up by gaining exp by knocking out opponents and by doing missions. There are 5 main archetypes for coh, and a different 5 for cov. I do not play cov at all, so I do not know what those classes are. In coh:
Blaster Damage High Hitpoints Low Primary powerset ranged attack Secondary set Support
Blasters are your glass cannons. They do very high damage, but cannot take a hit
Controller: Damage Low, HP low, Primary set Holds Secondary set Buff/deBuff
Controllers will hold an enemy so they cannot move, cannot attack, cannot see.
Defender: D Med HP Med PPS Buff/deBuff SPS Ranged attack
Defenders heal and help other people fight, though they themselves can fight when necessary.