The Basics of Photography

This is one of the best explanations of how aperture, shutter speed and sensitivity (ISO) interact to create exposure.

http://www.pbase.com/wlhuber/the_basics

I highly recommend it. The metaphor of a see-saw makes the concept easy to grasp and memorable. The article and illustrations provide solid answers to common questions new photographers ask, such as is there a relation between ISO and shutter speed or aperture? The answer is yes, and the see-saw makes this relationship visible.

It is helpful to keep in mind that a combination of shutter and aperture (at a given sensitivity)
has equivalents resulting in the same light value. So that 1/125 sec. @ f/11 is the same exposure as 1/250 sec. @ f/8. Increase the shutter speed by one stop and you need to decrease the aperture number by one stop (the aperture opens wider to allow more light in to balance the reduced time the shutter is open). The calculation can be verified at Bob's exposure calculator. Select 14 for existing light, ISO 100 for sensitivity, then f/8 and f/11 on the aperture priority side or 1/25 and 1/250 on the shutter priority side to see the change.