Exposure is one word for describing how much light the digital camera sensor is exposed to. There are two factors that set the exposure: the aperture and the shutter speed. Exposure is also described as one number called the Exposure Value (acronym EV).
The two most important things to remember when taking digital photos is composition and lighting. One of the way to control the lighting in the photo is by setting the camera exposure. Setting the Canon Digital SLR to a wrong exposure value will result in digital photos that are either over or under exposed.
The exposure is controlled by two factors: the aperture and the shutter speed. The aperture setting controls how wide the lens opens up when the photo is being taken. The shutter speed defines for how long the digital camera sensor is exposed. Each combination of aperture and shutter speed defines a certain exposure value or in other words defines how much light the sensor is exposed to. Since the exposure value is controlled by a pair of numbers there is more than one combination that results in the same exposure value. Although from lighting perspective all those combinations are the same from other optical perspectives, for example the depth of field, they are different.
It is important to emphasize that the exposure value is a number that represents to what extent the Canon Digital SLR CCD sensor is exposed to light as opposed to how much ambient light is there in the scene. The exposure value number does not change with the scene lighting changes it is always the same even in complete darkness.
The exposure value units are arbitrarily defined. It is a standard that exposure value 0.0 is defined as the exposure when aperture is set to f-number 1.0 and the shutter speed is set to 1 second. Other exposure values are relative to those settings.
Usually you are not required to directly set the exposure value. The camera automatically calculates the exposure value needed based on the ambient light measured. It is more common though to set the aperture or the shutter speed or sometimes both. Some features in the Canon Digital SLR are defined in exposure value units. For example exposure compensation or auto bracketing settings are defined in such units. Exposure compensation is basically telling the camera to lower or raise the exposure value when taking photos to compensate for errors in ambient light measurements, shades or just to get a specific lighting effect in the digital photo. When setting the camera to exposure compensation or auto bracketing you will have the option to define how much should the exposure value change. Usually exposure compensation does not change in more than five units.
Auto bracketing allows you to tell the Canon Digital SLR to take a few photos each time you press the shutter, each photo with a slightly different exposure value. You can also define the steps in which exposure value is changed. For example if you are not sure what is the right exposure setting, set it to the best you can or let the camera set it automatically and then set auto bracketing to take extra two photos or each photo taken one with exposure value lowered in 1 unit and one raised in 1 unit.
This article and more are from Danette Mckay who is an expert in his field. photo printing provides more in depth information.