Vsynch is Vertical Synchronisation.
Your CRT and LCD monitor run at certain refresh rates... take for example 85Hz, a common refresh rate.
That means the screen updates itself 85 times a second, REGARDLESS of what the graphics card can push out.
So if you have a kickass graphics card you often get more frames per second than say 85.
So if Vsynch is on, the extra rendered frames are discarded, and only 85 are shown per second, matching the monitor and giving a clean smooth image.
Without Vsynch, the graphics card pumps out as many frames as it can, and you get frame updates half way through the monitor updating the previous one. This can lead to frame tearing (odd flickering) and a reduction in image quality.
Personally though i havnt had much issue with Vsynch off. its a personal prefrence thing. Try it and see.
<b>Regards,
Mr no integrity coward.</b>