It has 4 different characters to play with, and they have each a unique skill, and as you level up you get a point to spend in one of 3 specilization trees that each character has.
For example my favorite character Mordekai can specilize in either sniping, using a revolver or improving his pet hawk.
There's also a very good loot system. You get different guns, shields, skill mods, grenade mods, etc...
You can level up in both singleplayer or through a co-op campaign with friends or random strangers.