If you are shooting with a recent Canon DSLR, you will notice that you have three autofocus modes: One-Shot, AI Servo, and AI Focus. Here is a brief explanation of what each of these modes does:
One-Shot. Use this mode when your subject is stationary. I use this mode most often when shooting landscapes or product photography in the studio. Sometimes this mode is called single autofocus. When in One-Shot mode, the camera will lock in a focus point when the shutter button is pressed halfway and will hold that focus point until you either release the shutter button or press it all the way down and take a picture.
AI Servo. AI Servo focus mode is most useful when photographing moving subjects, like sport, or a race. In this mode the camera locks on a focus point when the shutter button is halfway pressed, but continues to monitor and refocus automatically if the subject or camera is moving.
AI Focus. AI Focus mode combines One-shot mode and AI Servo Mode. In this mode, the camera will automatically switch from One-shot mode to AI Servo Mode if the subject or camera starts moving. Canon documentation states that this mode is great for photographing a subject that might move unexpectedly, like a child or wildlife. (Same thing right?)
I find myself shooting in One-Shot or AI Servo focus mode the majority of the time.