If you use VSync, it forces a full frame of graphics to your monitor before posting. This resolves some of the "tearing" that can be seen at times. There is a discussion of VSync in one of the other threads in this forum.
A couple things you should know about VSync. If you are using an LCD monitor, set the game FPS to 60 and enable VSync. Since the default refresh rate of an LCD is 60 Hz, they sync up nicely (unless you've changed it in your monitor settings, in which case change it back).
If you have a CRT monitor (old tube style) and have a refresh rate higher than 60 Hz (which we all used to do to get rid of the flicker), then you probably don't want VSync. If you monitor's refresh rate is higher than the game FPS, then your monitor has to wait part of a cycle to complete a full graphics page before posting it. The result can be an actual slow down of the video.
You can get around this if you have a monitor refresh rate 2X the game FPS (say 80 Hz for the monitor and 40 for the game), or set both to 60 FPS and 60 Hz.
For me, I saw a nice improvement with my LCD setting the game FPS to 60 and enabling VSync... everything looked "cleaner."