There was another website by a guy who does a lot of night pictures in Cambridge, but I lost the link. If I find it, I'll post it. His site has a lot of good information, and a lot of stuff that goes right over my head.
Lots of links provided for Histogram, so I won't repeat them. As I understand Histogram, (and here is where I will find out from others if I got it or not :^)), it is basically showing how much light you have gotten to your sensor ranging from the left side of the histogram, (shadows), to the middle, (midtones) to the right side, (highlights). In other words, it's the light exposure values represented graphically for each shot you take.
So the jaggy looking histogram display should match up to the exposure itself when you look at it in your image editing software. What I mean is if the histogram is ramped up in the highlight portion, you should have bright highlights in the picture. If the histogram is low in the shadows section, you'll get low, pixelated detail in the shadows. Therefore, the recommendation is to push exposure in digital cameras to the highlight side of your histogram display. That means slower shutter speeds for any given aperture.
The histogram can show if you are burning out your highlights. What that means is the ccd sensor in your camera is linear in its absorbtion of light. When it gets too many photons into a pixel in the sensor, that pixel burns out, (goes white, it doesn't damage the sensor. That portion of the image is overexposed). In your image editing software, that highlight won't have detail. If it happens to a lot of pixels, you loose lots of photographic detail. The same happens if the histogram displays a very low ramp up, (underexposed).