"Vsync is used to synchronize the output of your graphics card with the display of your monitor. When your graphics card has finished rendering the next frame it waits for the monitor to finish displaying the current one before switching to the new one. This means that the maximum framerate you can obtain will be equal to the refresh rate of your monitor (which is usually 60hz, 75hz, 85hz, or 100hz)." (
http://www.fraps.com/faq.php)
I use no sync so I get better performance but it depends on your computer hadware and what it is capable of.