Most camera also allow you to set an exposure compensation, refereed to as exposure value (Ev).
Exposure Compensation is a feature of a camera that allows you to adjust the exposure measured by its light meter. Usually, the range of adjustment goes from +2 to -2 Ev in 1/3 steps; or +3 to -3 Ev in 1/2 steps.
This means that you can adjust the exposure measured by the light meter by telling the camera to allow more light in (positive exposure compensation) or to allow less light in (negative exposure compensation).
Depending on how your digital camera is programmed to handle exposure compensation (and the mode (Av, Tv, P etc)), the camera may change the aperture while maintaining the shutter speed constant; it may change the shutter speed while maintaining the aperture constant; or, it may change both the aperture and shutter speed.
The viewfinder may look like this...
Why would anyone want to set an over or under exposure on purpose?
Because there are certain situations where the light meter of your digital camera can be confused.
As an example, say you are taking a scene where there is an plenty of light around your main subject (e.g. at the beach on a sunny day, or surrounded by snow). Using Weighted-Average metering or Multi-segment metering, your camera might be 'deceived' by the copious amount of light and expose for it by closing down the aperture and/or using a faster shuter speed (assuming ISO is constant), with the result that the main subject is under-exposed. By setting a positive exposure compensation, you are making sure that your main subject is correctly exposed -- though the surroundings would be overexposed.
A second example at the other extreme where the background is too dark, and the camera exposes for the lack of light by either opening up the aperture and/or using a slower shutter speed (again assume ISO is constant), then the main subject is over-exposed. By setting in a negative exposure compensation, you would in fact be under-exposing the surroundings, but properly exposing the main subject.
Bracketing The Exposure
There are many other situations where setting a positive or negative exposure compensation is useful. Many photographers will bracket their exposure when taking an important shot by taking one shot using the camera's measured exposure, a second one at positive exposure compensation, and a third one at negative exposure compensation. This is called "bracketing" and is a good approach to use when you need to make sure you get a good image in a difficult to expose situation.
You can combine the three bracketed images into a single image using software in a technique knows has HDR (High Dynamic Range) processing. But this is beyond the NTP scope.