When would you use a pie chart rather than a bar chart?

A pie chart can only have one series. So you are usually showing how a block of data is split up, like slices in a pie, and how much each element contributes to the whole lot. So you could have the population of a country and split it up into male and female. The full pie is the whole population and the two slices are how many males and females there are. A company could have its total spending, and in slices show how much goes on different aspects of what they spend it on. Slices can be shown with values or with percentages.

Bar charts are used for more than one series normally, though they can show one series only too. They can be used to compare data. The data does not have to be directly related, which it is often showing separate series of data. So it could be showing students and how much they got in an exam, comparing this against their other exams or against other students.