The GeForce Tweak Utility guides can be found in the GTU helpfile, which you can access at Start Menu -> Programs -> GeForce Tweak Utility -> GTU Help when you've installed the full GTU package.

How to use the helpfile:
Klick this button

and then click on a setting under the System / Direct3D / OpenGL tab.

You will also find this on several tabs:

Press it to get more information about the whole feature this tab represents.