!>http://www.livingroom.org.au/photolog/images/thumbnails/histogram.gif! "Photoblog":http://www.livingroom.org.au/photolog/tips/histogram_tips.php. These things appear everywhere, but what are they exactly. Photoblog has a good definition cribbed Photoxels:
bq. The histogram is simply a graph that allows you to judge the brightness of an image. You can think of the area under the graph as comprising all the pixels in your captured digital image. The left side of the histogram depicts how many "dark" pixels you have captured; the right side, how many "bright" pixels you have captured.'
"Outbackphoto":http://www.outbackphoto.com/workflow/wf_41/essay.html has a good overall description of what you are trying to do with the histogram. In short, you don't want it to pile up to the right (over-exposure) nor to the left (under-exposure). The shape doesn't matter as long as it is mainly in the middle.
You also want to check each individual color as you could get one color blown out. In this histogram, everything looks fine, but the blue is way to the right (overexposed) because the photo was of a blue flower. Not that in the average histogram of all colors (red, green and blue), green gets valued the highest, so even if the overall histogram looks good, it could be that red or blue or overexposed. The traditional histogram is also called the luminanc histogram FWIW.