For Canon system, one shot allows you to semi-depress to focus and locks the focus point for you to recompose the frame.
AI servo is the predictive focusing, which continuously focuses on your subject if it moves.
AI focus is a one shot focus that automatically switches to AI Servo if the subject moves.
AF-A = auto focus- auto, camera will determine whether to fix focus or to go predictive.
AF-S = AF-single locks on object
AF-C = af-continuous, tracks the object as it moves and focus adjusts predictively