These letters follow a formula:
First sentence states that one is pleased to write a letter of recommendation for [individual's name]. Obviously, it is important to have your name at the top of the body of the letter. The rest of the paragraph describes how long the writer has known you, the circumstances under which you first met, and whatever interactions you've had. (e.g. that he was your professor and later you served as a teaching assistant or lab assistant). If you've had continued contact after your regular meetings ended, that gets mentioned too (some people stay in touch for years with an advisor or mentor).
The next paragraph describes the activities in which you engaged as observed by the writer. Some writers will go on & on about what they cover in a course or the type of research they do in the lab. Some writers will cover how the applicant did grade-wise with quizes, tests, assignments or how much the applicant participated in class discussion or the topic and quality of a big written assignment or class presentation.
[Optional: a paragraph describing other activities that the writer knows of - because you told him - but that he didn't observe himself. It might open by saying that the applicant has been active on or off campus, etc, etc., or has been drivien in preparing for admission to med school (that's not too flattering if taken the wrong way) or something along those lines).]
The next paragraph covers a subjective assessment of the applicant's personal characteristics.
The final paragraph is a closing stating that the writer supports the applicant's application to medical school and usually says something nice about wishing that they were going into graduate school but they are well suited for medicine or that the writer would be happy to have the applicant as a doctor some day. Most end with something saying if you wish to speak to me you may call 000 000 0000.
