[wulf]

The most useful explanation I've come across is to think of a triangle. The area of the triangle is the rate at which incoming light is turned into the final picture. The three sides of the triangle, all of which can be adjusted to affect the overall area, are film speed (ISO), aperture (f/stop) and shutter speed (in seconds or fractions thereof).

The science of taking a picture is based on judging how much light is available and then adjusting the parameters in your control to get a resulting picture that has a suitable spread of tones and doesn't lose information in black (no light captured) and white (too much light captured) areas.

The art of photography is understanding the side-effects of adjusting each of the settings and making a decision that results in the image you intend. Tiberius' tutorials have some good information on this. The best way to grasp it though is to experiment yourself - take pictures, adjust settings in a methodical way and examine the results.

Wulf