The main difference between a histogram and a bar graph is that a histogram displays quantitative data while a bar graph displays qualitative data. Quantitative data is numerical and can be measured by counting. Qualitative data refers to a trait or characteristic.
An alternate way of expressing the way bar graphs and histograms display data is that bar graphs display nominal data while histograms display ordinal data.
Another difference between histograms and bar graphs is the order of the bars. In bar graphs, the bars are arranged in order of decreasing height. In histograms, the bars are arranged in the order that the classes occur.
Bar graphs are similar to pie charts in that they both display qualitative data. In a pie chart, the data that represents a trait is displayed as a piece of a pie. Each piece makes up percentage of the pie and all data categories combined equal to 100 percent of the pie. When deciding on whether to use a bar graph or a pie chart, it is usually recommended a pie chart be used. Bar graphs allow the user to better discern the difference between the traits. The heights of the bars are easier to distinguish than the wedges of a pie.