This is quite funny really- i didnt dare to ask the question so lad someone else did :
The difference between a coop and a multiplayer is that the classic multiplayer you can do with one or more people but even 1 person can clear all camps if they so choses.
On a co-op the map is divided in different sectors and only one player can clear that sector (the escape is you can use buffs on all camps on the map so you can "cheat" a co-op's intention.
The co-ops were created to force players to do real multiplayer gameplay because otherwise the co-op maps are pretty much undoable. The classic multiplayer youc an solo to your hearts content