A histogram is actually a bar chart, used to tabulate frequency of incidents relative to subjects. It's commonly used in statistics, and is a familiar illustrative tool in business graphics. Its most effective function is to provide an easy access of information, rather than using a table or chart. There are two basic types of histogram, the simple frequency based version, and a modification of that form known as a cumulative histogram, which produces a cumulative total. The weak point of histograms in some cases is their ability to express certain types of information clearly and in some cases "binning" of categories together, instead of separately, where the individual statistical properties can be better expressed. 
Examples of Histogram:
http://en.wikipedia.org/wiki/Histogram http://fourier.eng.hmc.edu/e161/lectures/contrast_transform/node3.html http://www.math.uah.edu/stat/objects/devices/Histogram.xhtml 
Histogram of travel time, US 2000 census. Area under the curve equals 1. This diagram uses Q/total/width from the table. 
Histogram of arrivals. 
