Histograms are a topic that we could (and probably should) spend a lot of time talking about but let me give you a very brief answer to get you through in the short term.
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. 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.
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 now you know what a histogram is – grab your digital camera’s manual and work out how to switch it on in playback mode. This will enable you to see both the picture and the histogram when reviewing shots after taking them.
Keep an eye out for histograms with dramatic spikes to the extreme ends of either side of the spectrum. This indicates that you have a lot of pixels that are either pure black or pure white. While this might be what you’re after remember that those sections of the image probably have very little detail – this is a hint that your image could be either over or under exposed.
The histogram is really just a tool to give you more information about an image and to help you get the effect that you want. Having your camera set to show you histograms during the view process will tell you how your image is exposed. Learning to read them will help you to work out whether you’re exposing a shot as you had hoped.