Most digital single lens reflex cameras have three focus modes: One Shot, AI, AI Servo.
One Shot focus
One Shot is the most basic DSLR focus mode. Push the shutter button down half way to lock focus. It will not change or refocus until you lift your finger completely and repress the shutter button. This mode is useful for general photo taking such a scenes.
AI* focus acts the same way as One Shot but is more intuitive. If it senses something moving, or if you move, AI switches the camera into AI Servo mode. Most seasoned photographers, particularly wildlife photographers, recommend using AI Servo instead of this mode.
AI Servo, also known as predictive autofocus, is active the entire time the shutter button is pressed down. A moving subject will be followed by the active focus point and the lens will continuously autofocus.
This mode is used when taking action shots and a tracking a moving subject. It is especially useful when taking photo using the high frames per second drive mode (burst mode).
AI Servo is one of the main reasons I decided to buy a digital single lens reflex camera. Coupled with a good lens, it will be very useful when taking wildlife photographs.
Effectiveness of AI focus modes
Several factors influence the effectiveness of AI focusing. They include: how close you are to a moving subject, how erratic it’s movement, the brightness and contrast of the subject, and the lens used. And, of course, if you don’t lock focus correctly from the get-go, you’ll get nothing in focus at all. So if you don’t lock focus right the first time, keep tracking until you do lock focus.