how to read the histogram

does the histogram on the 20d show you when you have a correct exposure? I have been told that the peak needs to be in the middle. What if it is to the left or the right. Thanks

It doesn't matter where the peak is. What you don't want is a peak that is cut off at either the far left, or, far right.

Broadly speaking, brighter images will have peaks on the right and dark images to the left. But you may have some images, like that of snow where you need the peaks on the right. So peaks need not be in the middle though it is generally true.

What you should really look for is peaks at the extreme end of the histogram which is a definite case of loosing some information because you have let in too much light.

or too less light

Here is some information:

Understanding Histograms
A Practical Guide to Interpreting RGB Histograms