People use pie charts and bar graphs as two ways of representing data in a visual format. Both formats have strengths and weaknesses with regards to displaying data and information.
Graphic Difference

Pie charts are circles that are broken into different slices based on the percentage of each category, while bar charts use an x/y axis and columns to represent different data. The bars can be either horizontal or vertical.
Calculation

Bar charts represent raw data. For example, if one category's total is 40,000, then 40,000 is shown on the graph. Pie charts represent percentages of a whole. For example, if a category total equals 40,000, but 40,000 only accounts for 10 percent of the total, that category will make up 10 percent of the circle.
Time Frame

A bar graph can show change over time, while a single pie chart cannot, only representing the given percentages at a fixed point in time.
When to Use Bar Graphs

Bar graphs should be used when showing quantities that are either not necessarily linked or demonstrate change over time. For example, use a bar graph to show either company revenue totals over the past decade or raw sales figures for different departments.
When to Use Pie Charts

Pie charts should be used when the data shows how different categories relate to the whole at a particular point in time, such as either company expense categories or percentages of city statistics in a particular year.
References
 Photo Credit Ryan McVay/Stockbyte/Getty Images