The shutter speed, f-stop, and ISO rating. These three guys work together to make sure that your image is exposed correctly--not too bright, and not too dark.

In fact, all of them can make the image darker or brighter, and the camera modifies another one to compensate. (Unless you're in manual mode, and then it's all you!)

This toy allows you to mess with these settings and see what happens. Notice how the background gets more or less blurry. Notice how the car gets more or less motion-blurred. Notice how the background gets more or less grainy. Finally, note how the camera compensates when in Av or Tv mode.

(I know, the actual numbers for shutter aren't particularly realistic unless the sports car is moving very very slowly. But the point is still valid, I swear!)

Just grab the sliders and watch what happens! When you're done with that, click on the links on the right side of the demo for brief tutorials.