Digital Grabber wrote an article on how the aperture and shutter speed features on your cameras work:
Shutter speed and aperture values are at a constant tug-of-war concerning light. Suppose someone was a handed a camera and was told that its aperture and shutter settings were set to ensure proper exposure using a shutter speed of 1/60th and an aperture of F/8. However, suppose that they wanted a faster shutter speed to capture a moving object, so they set the shutter speed to 1/250 and took a picture, they would get an underexposed photo because they didn't allow the film to be exposed for the 1/60th of a second it needed..
Read on over here to learn more.