What's the difference between a histogram and a bar chart?

A bar-chart is a graph where the height of the bar measures the frequency of a particular category which that bar represents.

For a histogram, however, the height is the frequency density. This means that the area of the bar is the frequency. These are usually used when the category is a range (e.g Ages 5-10, 10-15 and so on).

For histograms: area = frequency = frequency density x width of the bar's category.