Whether you're looking to brush up on your own skills and knowledge before picking up a dSLR or trying to teach someone else, there's little that beats hands-on experience. Of course, it's not always practical to go outside and practice.
The CameraSim dSLR simulator is a nice alterative. It's a flash based dSLR simulator that lets you adjust settings, take a picture and see the results. You can play with the ISO, aperture, shutter speed, focal length and distance from your subject.
Nothing beats actual hands-on experience of course, but it's not a bad way to go. You can check out the simulator yourself at CameraSim.com