Histogram: Darrell Young summarizes: "Your histogram is simply a graph that lets you see at a glance how well your image is contained by your camera. Too far left, and the image is too dark, too far right, and the image is too light". Read more in his excellent article Understanding your Digital Camera's Histogram
Michael Reichmann calls it "possibly the most useful tool available in digital photography". He explains what a camera histogram is, what it tells the photographer and how best to utilize that information in his article Understanding Histograms,
Exposure: Getting the right exposure is essential in photography. Moose Peterson writes that the "perfect" exposure is subjective and depends on what the photographer wants to communicate: This was written in 1995 for film cameras, but applies to digital cameras just as much: What is "the" perfect exposure?
Michael Reichmann found out that you get better results with your digital camera when your histogram is as close to the right side as possible. This is due to the nature of the digital image sensor. Read more about it in "Expose (to the) Right"
Canon: Do you onw a Canon camera? There is a great interactive on-line tutorial for Digital SLR users by Rick Sammon: Digital Photography Basics and Digital Rebel XT lessons
If you are a Nikon D50, D70, D80 or D200 user, check out the on-line tutorial, which covers all the SLR camera's features: Nikon Digitutor
The best way to become a true expert is of course to study the masters. Check out the books and videos from the great masters of photography below: