Difference between Black Ops 2 and Black Ops 2 Multiplayer?

Black ops 2 is the game itself, black ops 2 MP is just a little file expansion.

You'll need both to play the MP.

You will need both to play MP but you will only need "Black Ops 2" for SP

You dont need to download the Black Ops 2 campaign, i just downloaded MP and played it, then i downloaded the campaign but unninstalled it cuz i dont have too much hard drive space for bf3

Actually 3 black ops black ops mp and black ops zombies ;P and you have to install all to play any of them unless they have changed it since the game was released

Most Call of Duty games are developed like this. The Campaign, Multiplayer and other modes are separated so much that you need to completely exit one to play another. On a console you even get a loading screen during the changeover. I've seen this in other games as well, like Halo and Mass Effect.

for the previous cod you needed both multiplayer and campaign installed to play anything but in blackops 2 you can play multiplayer without zombies or campaign installed