One of the main advantages of digital photography over film photography, at least in my mind, is the ability to view the histogram shortly after taking the shot. While viewing the image on the LCD is obviously an advantage as well, it’s difficult to tell if the shot actually turned out based on a small image on a 2-3″ LCD.
An example histogram from Luminous Landscape
A histogram shows you how many pixels in the image (shown on the vertical axis) are at each intensity level (0-255 typically, shown on the horizontal axis). If the histogram is bunched to the left, that typically means the image contains mostly low intensity values, and is probably underexposed. If the histogram contains a tail that runs off the right-hand side, that typically means there are pixels in the image that are overexposed. You can typically recover information from the shadows, but it is impossible to recover information that is overexposed, so it is commonly known in photography that it is better to underexpose than it is to overexpose.
Due to the digital nature of image capture on a CCD or CMOS, a lot more information is recorded at large intensity values than at smaller ones. Therefore, a rule of thumb in digital photography is to attempt to get the histogram as far to the right as possible without forcing the tail off the right hand side.
The histogram is a very valuable tool for anyone who is serious about photography, since it allows you to quickly determine if the image was over or under exposed. Almost every digital camera has the ability to show this information, so if you aren’t familiar with it, you should check your camera and familiarize yourself with it.