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).
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
Click here to visit 123di.com