Hello,

I am running a Intel 2500K oc at 4.0GHz with Nvidia GTX680. I also have a 120 Hz monitor.

I sometimes see tearing of the picture. I thought that this had been eradicated by setting VSync to on. Am I missing something and is there some other setting/s which I need to do.

Nevica

Shouldn't be getting any tearing even with vSync off if you are running at 100-120 fps (lower ingame settings), that's supposed to be one of the big strengths of 120hz.
But if vSync is fixing the tearing and not getting you any input lag, then yeah just use that

So what could the tearing be caused by please? How do I monitor FPS in game please?

I presume Vsync will limit the FPS to some set FPS. How does it work please. Is it better to turn VSync off?

Nevica

Most people just use FRAPS to monitor FPS (and record gameplay, and take screenshots). Really small program, free download and no setup required, just have it running in the background and it will display an FPS-counter in the corner of your screen in any game you play (can be moved around and toggled).

http://www.fraps.com/download.php

vSync limits the FPS to specified fractions of your monitor's refresh rate i.e. 100%, 75%, 50%, etc to stop screen tearing. So with vSync turned on you will specifically see either 120fps, 90fps, 60fps or 30fps - whatever framerate you are getting at the moment, vSync will lower it to the closest "fixed" amount (so for example if your actual game is running at 82 fps, vSync will fix it at 60).
Occasionally vSync bugs-out with some games and locks it at 60fps the whole time, which would render your 120hz monitor a waste. That's where FRAPS comes handy

Most gamers tend to disable vSync unless the tearing is REALLY bad, or they are confident that their FPS will never drop below their monitor's max refresh rate. It can also sometimes introduce a small amount of input lag, especially if you have Triple Buffering enabled as well. Triple Buffering allows vSync to split into further fractions, e.g 120fps, 105 fps, 90 fps, 75fps, etc.

You really shouldn't be seeing any tearing and shouldn't need vSync if you can run at 100-120 fps minimum, which is easy with a GTX680 if you drop to Medium settings.
I'm running a similar setup with i5 2500K and 120hz monitor, but with a 780.