In the film, Jolie plays Evelyn Salt. She works for the security services. One day a former KGB agent walks into her offices, and claims that there are Russian sleepers dotted throughout the security services. He adds that he knows because he put them there and that Salt was one of them.
The remainder of the film is an examination of whether Salt's a spy. There are all the explosions, shoot outs and car chases that you'd expect from this sort of film. During this other sleepers pop up left and right, leaving you with the feeling that there are almost as many Russians as American in the upper echelons of the American Secret Service.
All in all, it was pretty standard fayre for this sort of film, but it was pulled off with enough elan and "flair" to make it work watching all the same.