Histograms are a special form of bar chart where the data represent continuous rather than discrete categories. This means that in a histogram there are no gaps between the columns representing the different categories. In the example below, a histogram has been used to show the average height of children of different ages in 1837. A histogram is used because age is a continuous rather than a discrete category.
In a bar chart the length of the bar indicates the size of the category, but in a histogram it is the area of the bar that is proportional to the size of the category. This difference is due to the fact that in a histogram both the x-axis and y-axis have a scale, whereas in a bar chart only the y-axis has a scale.