They both look very similar and can be confusing. Essentially histograms are a type of bar graphs. Both these charts have frequency on the Y (vertical) axis and categories on the X (horizontal) axis.
In a bar graph the variables on X axis are arbitrary. For example if you ask one hundred 8 year old students to pick their favorite ice cream flavor the result can be plotted as a bar graph as follows. Note that the bars do not touch each other.
Bar graph
On the other hand in a histogram the variables on X axis are part of a population and the data on Y axis is numerical. For example say we measured the heights of the same 100 children. We could plot the graph as a histogram as follows. The X axis has range for height in inches. Note that the bars touch each other. In the USMLE you might be given some data and asked what kind of a chart will be most appropriate to present that data. With increased emphasis on bio-statistics and evidence based medicine, interpretation and presentation of such data may be tested more frequently in the USMLE.
Histogram
Click biostatistics label on the label cloud to view more posts on statistics in the USMLE
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