What is exposure?
Wikipedia (http://en.wikipedia.org/wiki/Exposure_(photography)) states the following:
In photography, exposure is the total density of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance over a specified area.
There are three elements which affect the exposure of a shot.
3. Shutter Speed
Each of these elements changes the way light hits the film/digital sensor. A change to one element will affect the remaining two. Therefore when adjusting one element you will always need to consider the effect on the others. Modern digital cameras do provide assistance by allowing manual control of one element while automatically adjusting the other two for a good exposure.
Mastering exposure takes a lot of practice. The best way to learn is to take lots of pictures with different settings and study the results. The great thing about digital cameras is you can take as many pictures as you like at zero cost.