The default mode on Canon DSLRs. Locks focus when half-pressing the shutter button. Useful for most situations (still life, portraits, landscapes, etc.)
AI Servo mode will keep a continuous focus on a moving subject. Useful for any shots where the subject is moving, such as pets or for sports. Using continuous focus will drain the battery faster than One Shot mode, as the camera is working harder in both hardware and software.
AI Focus is a hybrid between the two previous autofocus modes and is the default on cameras that have this capability. AI Focus will default to One Shot-style focus but will autofocus if the subject moves.
Focus is adjusted manually via the lens. The camera will not assist in focusing and relies entirely on the eye of the photographer. Very useful when autofocus does not behave as desired.