Courtesy of a fav site of mine, clear concise information on a range of topics. And there are usually sample images (rollovers with your mouse for comparative purposes) to go along with the text:

Camera Histograms - Tones & Contrast

Camera Histograms - Luminosity & Color


Image Editors.

To bring things together once you are in the editing chair, and if need be.

Photoshop.

How To Read An Image Histogram In Photoshop

Paint Shop Pro.

Histogram Adjustment - Using Paint Shop Pro 9.0

Using the Histogram tool in Paint Shop Pro X2