The historgram is a visual representation of the frequency with which something (or certain data) occurs, so the vertical axis shows the frequency of the occurrence, while the width of the bars show the values being measured. So you see the difference between this and a traditional bar chart, for example, is that the bar chart displays the values and frequency vertically instead, by the height of the bar, while the histogram shows values in the total area of the bar, so proportions are shown both in width (values) and in height (for frequency of occurrence).
