Terminator 2...what do i need to know?

i watched the first one...understood it. i dont have time to watch all of them before tomorrow so i got the 3rd one. what happens in the 2nd one? i know the terminator is on sarah conner's side now...but why?? he was just trying to kill them...now hes on the same side?? i dont understand..and whats the liquid man doing???? please help!