Understanding Histograms

What is a histogram?
A histogram is a tool that graphs the tonal range of an image.
What happens to the visual when a histogram is skewed to the left? Right?
When a histogram is skewed to the left, it might be overexposed. There are a lot of light tones in the image. When a histogram is skewed to the right, the image contains a lot of dark tones.
In most cases, what does a "good" histogram look like?
In most cases, one should strive for a balanced histogram with peaks in the middle.

Why is a histogram a good tool for photographers?
Histograms help show whether an image is over or under exposed and creating effect the photographer wants.