A bar graph is a graph drawn using rectangular bars to show how large each value is. One could use a bar graph to show how something varies over time or to contrast items as described here.
While a bar graph is visually easy to understand, the disadvantages, from this site, would be that graph categories can be reordered to emphasize certain effects and they can only be used with discrete data. Another disadvantage would include the fact that it is difficult to compare and contrast a large number of variables.
Bar graphs use one categorical variable which means that each bar represents a different aspect of the category. Here is a great reference to see this.