Why do you work, go 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. You are not working at a job because you get a great feeling of personal satisfaction from it or because it makes you feel good about yourself (that you are a good person), but rather to gain some kind of reward. We are not saying there is anything wrong with this. We are only trying to explain the concept to you.
See also: Intrinsic Motivation