I had an argument with a Steam friend, and the conversation was about the topic of so-called "multiplayer" games. I brought up games like TF2, Dota2, and SR3 as games that define what multiplayer is. My friend, however disagreed, and insisted that co-op and multiplayer are different categories; co-op, where you play along players, and multiplayer, meaning playing against someone else.
Am I wrong to think these are two seperate genres of games? Or are co-op and multiplayer virtually the same?