What's that strange-looking graph?

Cameras and editing software use a Histogram to display the distribution of tones in the picture. Is this important? Actually it's very useful—the histogram acts a bit like a graphical exposure meter. The shape of what I call the black 'mountain' is not as important as its position in relation to the baseline axes, that the mountain is sitting on. The left hand side of the graph represents shadows, the right hand side represents the highlights. The middle section represents the midtones (greys).

Graph A is from a very dark photo—a lot of the tones are slumped against the left hand (shadow) axis, resulting in detail lost in the darker areas of the picture.

Graph B is from a near-perfect picture, while Graph C is very overexposed - tones are pushed against the right hand axis.

You'll find this display in several places in Photoshop Elements— the Camera RAW...