Actually, the MvM (Mann Vs Machine) mode was added recently. Now, you have two modes:
PvP: Contains 32 players, 16 vs 16, and two teams: Red and Blu (Not Blue, it's Blu). Theres tons of mini-modes for PvP.
MvM: Contains 6 players, 6 vs ROBOTS. You and more five guys fight united against tons of...