Absinth pretty much covered it, but it could also include anything you've done to help the school, such as volunteering at parents' evenings, and it usually includes a paragraph from each of your subject teachers about your ability, as well as perhaps something about how you're well suited to your chosen course. If you have a look on the UCAS website, I think there's a guide explaining what the reference should contain, which would be useful to give to your referee.