This concept deals with three basic types of charts: bar, line, and pie.
A pie chart is useful when you want to show percentages. It clearly shows the relative proportions between categories.
A bar chart can show more information than a pie chart. The bars can be grouped by categories to show not only the relative proportions between categories, but also the relative magnitude of series data. A bar chart doesn't show the relative proportions between categories as well as a pie chart, but it is most useful when you want to also show the relationships between the series data.
A line chart is most useful when the X-axis represents time. It better represents trends.
The Insert/Chart... command in Excel allows you to experiment with different chart types. You can choose the type of chart you want based on the relationships in your data you would like to emphasize. You can then experiment with the different variations of charts to find the one that best conveys the point you are trying to make.