These amazing applets allows experimentation with camera settings in one of the three user control modes. (Shutter Priority, Aperture Priority, or Manual).
CAMERA SIMULATOR
This one provides realistic visual results based on your settings. Note the viewfinder adjusts to your settings. You can purposely overexpose or underexpose the image to get an idea of how to direct your camera settings.
http://camerasim.com/embed/camera-simulator/
VARIABLES THAT AFFECT EXPOSURE
Another realistic applet that is simplified, but shows appropriate blur, depth of field, and exposure based on selected settings. Live results after selecting the setting help to comprehend the exposure variables of ISO, Shutter, and Aperture.
http://graphics.stanford.edu/courses/cs178/applets/exposure.html/