|a digital meter scale|
|an exposure compensation button|
There is a setting on most SLR cameras called Exposure Compensation. This is a setting that will allow your camera to set its exposure, and then allows you to intentionally override the exposure by up to 3 f-stops in either direction (under or over expose) to get the desired results. With many of the early film cameras, this was usually a dial on the top of the camera that went from -3 to +3. With some of the newer SLR cameras, it was changed to a button.
|an exposure compensation dial (on a newer p&s digital)|
A good example of why you would want to underexpose and image would be if you were taking a picture of a full moon on a clear night. If you keep your camera on automatic, you'll see a bright light in the sky, but no detail in the moon. By underexposing by 2 or 3 stops, you'll see the detail and might just capture the best moon photo you've ever taken.
Another setting on your camera that was designed to help correct for these issues is called the Exposure Lock. This is typically a button that you push to lock in the exposure that the cameras light meter had taken. In order to do this correctly, you would have to walk up to the subject, push the exposure lock button, then while holding the button, walk back so that you could get the snow, or sunset back in the photo, and then take your picture. This button allows your camera to choose a setting based on the subject and not on the background, which will in turn let you correctly expose the area of your photo that is most important.