A histogram is a representation of all the light values within a digital image and allows the photographer to easily judge the brightness and contrast range of a photograph. In basic terms, all the points below the graph are all of the pixels within the digital image. The left side of the graph represents all of the "dark" pixels that have been captured, while the right side of the graph represents all of the "bright" pixels that have been captured. From to taking the original photo to making the final edits, understanding how to read and utilize histogram data is important for getting optimal results.
Histogram Overview
In general, a histogram is a graphical representation of how frequent data within a table fall into specific categories. In photography, a histogram is a graphical representation of all the pixel color values within an image. Picture a bar graph where the various tonal ranges from 0 to 256 are on the x-axis and the number each tonal range is represented on the y-axis.
References
Timothy Edberg Photo Tip: Digital Histograms Histogram Basics Histogram Tutorial Wikipedia: Color Histogram