The combination of aperture, shutter speed, and ISO setting controls the effect you get in your photograph. Ultimately it depends on the shot you want, and in any situation you someone may want to use a different combination than you, but knowing the basics helps. Now go read the manual and experiment with some of those extra buttons on your DSLR.

Shameless plug if you have a moment: Some of my photos on Flickr

Putting it all together: Exposure

The length of time the shutter is open, and therefore the light can reach the sensor.

Understanding shutter speed

The clever folks — Willard S. Boyle and George E. Smith — who invented the CCD sensor, the basis of what enables modern digital cameras to capture light and record a photograph were awarded the Nobel prize for their troubles. And the silver halide film Kodak were famous for is equally remarkable.

Understanding ISO