Bar graphs are just what they sound like: graphs that use bars to compare different quantities.
The bars are usually vertical, but it is possible to make a horizontal bar graph, too. Each bar represents a different quantity, and the height (or horizontal length) of the bar corresponds to the size of the number.
Bar graphs are used to compare the sizes of different categories of information. The labels on the horizontal axis are the names of the categories, and the heights of the bars show the sizes of each category. The bars may be in size order (in which case the graph is sometimes called a Pareto chart) or in some order that makes sense for the categories.
Bar graphs can also be used to show the change in some value over time, although a line graph is probably a better choice. In that case, the labels on the horizontal axis may be dates, months, years, or other time intervals.