VSYNC = Vertical Sync:
"VSYNC is a term used to describe how a graphics board renders and displays an image to the display. With VSYNC On, the graphics board draws one frame of a scene, and waits for the monitor to display it before starting the next frame. "
"With VSYNC Off, the board draws one frame, and then immediately starts on the next frame - whether the display is ready for it or not. "
-taken from a website. Best explanation I've seen yet...