I have always wondered about the use of histograms in the digital camera.. Akhiran I found a very nice article explaining it on the blog of Digital Photography school. very interesting.
Histograms are a very useful tool that many cameras offer their users to help them get a quick summary of the tonal range present in any given image. It graphs the tones in your image from black (on the left) to white (on the right). The higher the graph at any given point the more pixels of that tone that are present in an image.
The beauty of a histogram is that the small LCD display on your camera is not really big enough to give you an great review of a picture and you can often get home to find that you’ve over or under exposed an image. Checking the histogram can tell you this while you’re in a position to be able to adjust your settings and take another shot.
So a histogram with lots of dark pixels will be skewed to the left and one with lots of lighter tones will be skewed to the right.