using and understanding the histogram whilst taking images is a main key difference between a pro & amatuer photographer, along with being able to take control of the exposure and using manual focus instead of relying on the cameras autofocus mechanism.
on the Histogram it will show you the exposure (most of the graph is to the left = under exposed, to the right = over exposed) the amount of shadows, midtones and higlights, and if any colours are clipped.
this image below will illustrate the information shown, however an image may not be under or over exposed but shot deliberately as such to achive a Low key or High Key image
for this lecure our brief was to produce one low key, one midtone and one high key portrait picture. my images for this are below.