This is one of the most overlooked tools by a beginning SLR photographer. Most of the time your camera’s metering system will be very good about judging how much light is coming into the camera, however it isnt perfect.
Exposure Compensation is simple way to to “tweak” your camera’s metering system. So, say you are shooting in a certain lighting environment, and you notice your pictures are a little dark (see also your histogram). By sliding your exposure compensation up, you will tell the camera to “overexpose” the image according to how it predicts what it sees. (which means, ALL the pixels will be brightened by the amount you dial in). The numbers refer to F-Stops and the tick marks refer to 1/3rd F Stops.
An example, when shooting on snow, there is a ton of reflected light which tricks the cameras metering system into thinking it is very, very bright. The camera calculates this and underexposes the entire image. By experience I know if you shoot at +2 Exposure Compensation in snow, the image will actually be exposed properly.
Simply put, Exposure compensation allows you to brighten/darken your pictures in camera, before you take the picture.
A few side notes:
1. Exposure Compensation is mainly for Program Mode, Aperture Priority and Shutter Priority Modes.
2. The EC bar becomes a simple metering bracket in Manual Mode, because Manual Mode is essentially a form of Exposure Lock, therefore the bracket tells you how “exposed” the camera predicts the image to be.
3. You have to be sure that when you change your shooting environment to set your E.C back to “0”.
4. Keep a constant eye on your histogram, in addition to the image itself, it will tell you if you are over or underexposing.