Head tracking and eye gaze are both alternative access methods that allow someone to control a device without using their hands.
Head tracking is suitable for users that cannot access a device with their hands, but can maintain adequate head control for the built in camera to follow head movement.
Eye gaze uses specialized technology to track where a person is looking at the screen with more precision. It can be used for individuals with limited or no reliable head or hand movement, but can visually fixate and shift eye gaze accurately.