* long has he known you and in what context (as a classroom teacher and any other capacity including if you have stayed in touch if it has been a few years since you took the class)
* A few memorable tidbits about your classroom performance: participation in classroom discussion (lots, little but very thoughtful, not much but ...), memorable papers or other written material
* A paragraph about your professor's knowledge of your interest in medicine, and the attributes he has observed that would indicate that you would be a good physician.
* usually a summary statement that you are among the top X% of the students he has taught or that he would be honored to have you as his own physician, or that you would be a fine candidate for medical school and an endorsement along the lines of recommending or highly recommending or giving my highest recommendation "to ...insert name here... for admission to your medical school."