Co-Op is when 2 or more players co-operate to play against computer opponents in a story based missions. Online Co-Op is when 2 or more people co-operate online to play against computer opponents in a story based missions.
Not all online multiplayer games are not co-op and not all co-op games are online.
Usually offline co-op is implemented splitscreen (Halo1,2,3, Darkwatch(PS2) etc .. most fps and third person action games implemnt co-op as split screen, or shared screen Marvel Ultimate Alliance, Baldurs Gate Dark Alliance, Champions Of Norrath, Double Dragon 1,2,3, Streets Of Rage 1,2,3 etc.. Brawlers, Beat Em Up and RPG's are usually shared screen.
It is relatively simple to implement local splitscreen and same screen co-op. Online co-op requires tighter scripting and programming than what most studios can devote.
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