Among the most basic concepts in photography is exposure, yet it can be among the least intuitive concepts to grasp because of the way aperture, shutter speed and ISO settings combine to affect exposure. Many different textual and graphical representations of these relationships already exist; among the most popular is the "exposure triangle". These days, though, my favorite tool to help explain exposure is this great page from Canon:
The page has a "play" mode as well as a "challenge" mode, which is surprisingly helpful because it allows a new user to connect the "this is what I'm trying to do" idea with the "and this is how I do it" bits. I also love the fact that the app shows how Av and Tv work to move other settings for you in order to keep a correct exposure. Very handy, and a very nice learning tool. I'd love to see something like this in a portable app -- it would be a fantastic teaching tool in the field.