It's basically the same thing as in Single Player, complete the objectives, but there is another mode called Stopwatch, which is the same thing but this time you have just a certain time limit (it doesn't keep on adding more time every time the attackers complete a Primary Objective), that, and once the attackers are done they switch to defenders, the team that made the best time wins. The multiplayer is pretty cool in my opinion, just that it's hard to find a game with both teams full of online people. I suggest you watch some multiplayer matches first.