Autofocus Servo refers to the camera's ability to continuously focus on a moving subject, a feature normally only found on digital SLRs. It is generally used by sports or wildlife photographers to keep a moving subject in focus.

Autofocus Servo is normally engaged by switching focus mode to "AI Servo" (Canon) or "Continuous" (Nikon) followed by half-pressing the shutter release. The camera will continue to focus based on its own focus rules (and your settings) while the shutter release is half-pressed or fully depressed (actually taking shots). It is worth noting that Autofocus Servo normally also puts the camera into "release priority" mode so that the camera will take a shot when the shutter release is depressed, regardless of the current AF status (good lock or still searching).