How do you use the 'exposure' in digital cameras?

To make it real simple, consider what will happen when you point you camera at a white wall at the automatic setting:

1. The camera will meter the light bouncing off the wall and produce an aperture-shutter speed combination that is appropraite based on the light available. By default, the cameras try to average things, and so it happens that the average scene usually turns out to be 18% grey. So the camera will look at the wall and see it greyish, rather than white, as it acutally is. The solution? Override this automatic bias and tell the camera to let more light in, making the wall white. How you do that? Increase the exposure, if your camera has such manual override. Increasing exposure by 1 EV will double the amount that will come in. You never know by how much the exposure should be increased to get that perfect (rather than overexposed) white wall and so some cameras allow for what is called "exposure bracketing" -- the camera simply takes 3 pics rather than one (with different exposures each) and you then choose the best one later.

Of course no one really shoots white walls, but you get the point. Practical applications? Shooting a subject against a bright background, or any other situation where you should not trust your meter. Remember...the cameras want to see everything in 18% grey...