By pressing the right directional button () we can access the 'AF mode' menu, (AF is short for Auto Focus). There are three options here for changing how the camera's focusing system behaves: One Shot, AI Focus, AI Servo. Select the desired option with the arrow keys and confirm with the 'Set' button.
In this mode pressing the shutter button halfway obtains focus (indicated by the green circle in the viewfinder and the 'beep'. If we keep the shutter button half-pressed we can lock the focus and shot can be recomposed (if you move closer or further away from your subject it may become out of focus). Pressing the shutter button completly down takes the shot. This mode is best for still subjects.
In this mode pressing and holding the shutter button halfway will achieve focus, but not lock it. Instead it will constantly try to keep focus on the subject, even if the subject moves. The idea is to be able to maintain focus whilst tracking a moving subject. When focus is achieved the exposure settings are calculated and set, however there is no visual or audible confirmation. This mode is suitable for shooting moving subjects; animals, babies, sports etc.
How well this mode actually works is down to how fast your lens can auto-focus. If you're finding your lens is too slow at focusing on your moving subject consider buying an 'Ultrasonic' lens. Although costly, these lenses are very fast at focusing.
This mode is for when you cannot chose between the 'One Shot'and 'AI Servo?modes; The camera decides which mode is best to use depending upon the subject's movement. If focus is achieved in 'One Shot' mode then there will be the usual beep and confirmation light in the viewfinder. If focus is achieved in 'AI Focus' mode there will be no confirmation light, but there will be a soft beep.