A quick Google search revealed this:
http://www.silverlight.co.uk/tutorials/compose_expose/exposure.html
Theres probably even better resources out there if you search around a bit.
Thats for film, but the same concepts apply. The only difference is that you can explicitly set the ISO on your digital camera (unless your in full auto). A high ISO will increase the exposure of your image, but will introduce noise, or grain, into the photograph. Most people find this undesirable.
Your digital camera also likely has Programmable (or Automatic), Aperture Priority, Shutter Priority, and Manual controls. Programmable sets everything for you, you just choose the exposure of the image (either over exposed, underexposed, or correctly exposed; usually represented by a number from -2 to 2 on a scale on your cameras display). Aperture Priority allows you to set the aperture, and a desired exposure, and the camera will compensate. Same for Shutter Priority. Manual gives you full control over both the shutter speed and the aperture.
If you still have questions, then come back and ask.