What do the different auto-focus options mean?
One shot is best used for still subjects. In this mode the camera will only focus once. If there is any movement or change in the subjects distance or positioning it will need to be focused again.
In this mode, the camera will switch itself between One Shot and AI Servo depending on what the subject is doing.
This setting is best used for subjects where the distance is always changing (like sports). While you hold the shutter button down half way the camera will continuously focus.