Understanding exposure is very important.
Many people do no realize that shutter speed, aperture setting and ISO are interrelated.
i.e. if you go to a faster shutter speed, you need more light and therefore must adjust the aperture to provide more light.

Here is a tutorial that explains how they interact:

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

Here is a free short course in digital photograpy, that covers the important areas:

http://www.shortcourses.com/using/index.htm

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