AF modes — One Shot AF and AI Servo AF:

One-Shot AF: One-Shot AF means the AF system is set to focus upon a stationary subject, and then lock focus upon it once sharp focus has been confirmed. (One-Shot AF has nothing to do with whether you take one or a sequence of pictures.) If the user keeps his/her finger on the shutter release button, focus remains locked, making it easy to lock focus and re-compose to move a subject.

AI Servo AF: This AF mode sets the camera to follow-focus on a moving subject, whether it’s moving slowly, or at the speed of an Indy race car. The camera expects to see, detect and track subject movement whenever you’re in AI Servo AF.

There’s a separate button on top of the camera, similar to other EOS cameras like the EOS 60D or 5D Mark II, marked “AF•Drive”. Press this button, and then turn the top Main Dial to move between One Shot AF and AI Servo. There’s a third option, AI Focus AF, that lets the camera automatically pick between the two.

For a more in depth information on the EOS 7D Auto Focus System, please click on the following link: http://www.learn.usa.canon.com/resources/articles/2011/whats_news_eos7d_article.shtml