Tearing
It is an unfortunate fact of computer graphics that if you disable VSync, your graphics card and monitor will go out of synch. Whenever your FPS exceeds the refresh rate (e.g. 120 FPS on a 60Hz screen), or in general at any point during which your graphics card is working faster than your monitor, the graphics card produces more frames in the frame buffer than the monitor can actually display at any one time, so the end result is that when the monitor goes to get a new frame from the primary buffer of the graphics card during VBI, the frame may be made up of two or more different frames overlapping each other. This results in the onscreen image appearing to be slightly out of alignment or 'torn' in parts whenever there is any movement - and thus it is referred to as Tearing.