Aperture: http://en.wikipedia.org/wiki/Aperture

That is indeed the way to set the lens to F8, but it also means that the camera will need to find a shutter speed to match that aperture for the light conditions that you’re shooting in. That may not always be possible, or the best combination.

AF-S denotes the focusing system of the lens, see here: http://www.kenrockwell.com/nikon/nikortek.htm
AF-s or AF-c when set on the camera relates to single shot or continuous shooting, more info can be found here (one third down the page): http://www.kenrockwell.com/nikon/d50.htm

Based on your questions I would say that you could perhaps use a basic course in photography techniques and exposure. I would suggest doing some reading (books, camera manuals, internet) to get the best out of your gear.