This is too cool. A page that lets you play with shutter speed and aperture to see the effects in a photo. What a great way to get your head around some of thus stuff.
SimCam - Shutter and Aperture
All cameras have at least two ways to control light: the shutter and aperture. The aperture is like an iris in the eye, and controls light by changing size. When photographer takes a picture, the shutter opens briefly and the image is exposed.
In any shooting situation there are several combinations of shutter and aperture that could be considered correct. Which settings to choose is a matter of personal taste, and is usually influenced by how much light is available.