The variable that controls the rotation is updated every time a frame is rendered. If you move 0.1f per frame then at 100 FPS you move 100 * 0.1f = 10 rotations a second, if you can only manage 20 FPS then you'll move at 20 * 0.1f = 2 rotations a second. If you move too fast then you'll get jerky movement, simple as that :)

If you're just learning then it's nothing to worry about at the moment but if you want to 'fix' this then you need to time how long a frame takes to render and scale the movement by that. A different solution would be to separate the updating of the variable from its rendering and use a fixed update frequency.

Some timer code in Lesson 21

Hope that helps :)

No problems. Welcome to NeHe and the wonderful world of OpenGL :D

I'll point you over to the forums FAQ, it's got some excellent tips and shows you the correct way to post code segments, which as you're learning will be helpful I'd imagine ;)