The game either ends when all Framed are dead, which results in a police victory, or when the last surviving Framed escapes. There are also several other gamemodes, but they are very simple. You can learn how to play them in-game.
Now, about the overall experience of the game. It is very simple, but mastering it may prove to be a difficult task. My advice is to click as fast as you can when shooting someone. Oh, and you should never try to kill everyone you see. That will nearly always end terribly. If you kill someone that you are not supposed to kill (basically, someone who is not your target, hunter or an undercover, cops cannot be killed until there is only one Framed remaining) the cops would see your current location and they would be able to kill you, even if you do not pull your weapon out.