Focus Lock and AF Lock

AF-Lock: controlling focus
Gain more control over where the camera focuses when you depress the shutter-release button half-way. Say you want to focus on the subject that is off to one side in a scene. Move your digital camera so the focus area indicator is on the subject. Then press the shutter-release button down half way to lock focus. While holding the button in this position, recompose and then fully depress the button to take your shot. This helps ensure that the main subject is in sharp focus rather than a random object selected by the camera.

Focus Lock is very useful and I use it regularly. It can be used to pre-focus a shot ahead of time so that you don't have to wait for the camera to focus in an action-shot situation. I also use it when I am taking several shots of the same scene and don't want to bother refocusing each time. This is especially nice for low-light situations or other instances where the camera may have trouble focusing...this way you can get it right once and spend your time on other aspects of the shot rather than dealing with the focus.