Why do you work or come to class or study for a test? Do you do it because you want to money, a degree, and good grades? If so, you are extrinsically motivated - motivated to perform specific behaviors to achieve promised outside rewards or to avoid punishment from others. However, if you are working at a job because you get a great feeling of personal satisfaction from it, and you are trying to perform the behavior for its own sake (not for money), then you are intrinsically motivated. We are not saying that this is better or worse than extrinsic motivation, only different. Intrinsic motivation does seem to be more satisfying to people though. People who are extrinsically motivated tend to be less satisfied and become unhappy more easily (in general, not always).
See also: Extrinsic Motivation