Canon Cameras has a very fun, creative and helpful tool for understanding exposures on their website.
If you aren't exactly how changing f/stops (aperture), shutter speeds, and ISO settings will affect your photos, you can play with this tool and see the results, complete with explanations of what happened and why. It can also help you understand why to use Manual Mode, Shutter Priority, or Aperture Priority.
All of these apply no matter what brand or style of camera you are using.
Check it out at "Canon Outside of Auto."
Thanks to Scott Howard for pointing out this cool feature to share.