"At any time, your client is just predicting what it figures will happen. It doesn't know "for sure" what the state of the world is until the server tells it, it's just a best-guess until it gets the next authoritative state from the server.
So your client might be all "SWEET, dude, you just unloaded five MP5 rounds into this guy's hea-", but then the server's update will arrive that says "Nope. See, JohnnyStarZippo turned 23 deg to the left and sent an M16 burst into your skull. You've actually been dead for the last 1/4 of a second." And when you watch the killcam, that's what you'll see. It's an authoritative record from the server, a recording of what "really" happened.
Of course, the longer the communication time between you and the server, the more and more jarring the server's corrections will seem."
Rayme's Post