Auto focus mode

One Shot AF will focus on something and stop the AF process. One Shot AF will not fire the shutter until the lens is in focus with something. This AF mode is available in some PIC modes, like Macro mode, I think. AI Servo AF will not lock the focus, and if the subject or the camera moves, the camera will continue to seek a sharp focus. Servo AF will allow the shutter to fire whether in focus on not. This is mode only available in the Sports PIC mode. AI Focus starts out in One Shot AF, but if it detects subject movement it switches to AI Servo. It does not switch back to One Shot unless you release the shutter button and start over. This mode is available in P, Tv, Av, M and probably some PIC modes. The DRebel defaults to the AI Focus in most modes but switches very easily into Servo. Much easier than any other EOS camera I've used. In fact, too easily for most people. For example if the camera locks focus and you them move the focus manually (only on lenses with FTM), or sometimes if you zoom, or even pan the camera; all of these can force the camera to switch to Servo when you don't really want it to. The Wasia (Russian) firmware hack can force the camera to stay in One Shot, but it can't force the camera to start out in Servo.