V-sync problem

If you want an excellent explanation, take a look at the following article: 'Triple Buffering: Why We Love It'.

Basically, vsync forces your GPU to output a frame only when the display is ready to draw a new screen.
If it is disabled, you can tell your screen to render frames faster than it is capable of which results in tearing.

+1 to using Google (or even just the 'Search' option...).

Of course, since a moniter can't even display more then its supported Refresh Rate, I see 0 reason to leave Vsync off...

Using Tripple Buffering helps avoid several of the issues with Vsync, and should be used whenever supported.

http://www.anandtech.com/video/showdoc.aspx?i=3591&p=1

Explains most everything about Vsync/Tripple Buffering