As I've said before, I love my little Leica point and shoot camera. One of the cool things about it is that it shows the histogram on the LCD screen. When I push the shutter button halfway down to set the focus, several details, including the histogram, pop up on the screen. In the past, I've felt that I would not be interested in viewing this information. But I'm beginning to see a pattern when an image is overexposed or underexposed and how the histogram is correlating to this image. Jim Miotke explains the histogram as "a graphic representation of how each pixel in your picture is colored".
Here's one of Jim's images with a histogram overlaying the image.
Here's a short article on BetterPhoto.com that gives a brief description of what a histogram is:
Histograms: Not as Obvious Nor as Boring as You Might Think!
I had to pay attention to the histogram for some photos I was taking of the kids outside, in the later afternoon when the sun was bright and stong to one side. I let too much light get into the scene so my images were coming out completely overexposed. I saw the spike in the histogram and was able to reposition myself so that I didn't get too much light on Alex's face.