I am writing to recommend _[applicant's name]__ for admission to medical school. A statement describing the circumstances under which you met and how long you have known the applicant. If applicable: a change in the relationship should be described (e.g. applicant had been a volunteer and then became an employee or was a student and became a research assistant, etc)
Some writers will describe themselves, their credentials, experience in training others, scope of practice and/or research interests, current teaching methods or laboratory work. This can go on for a paragraph or two but is optional.
Objective description of what the applicant did. Interaction of the applicant with others. Description of the characteristics demonstrated by the applicant. Opinion of the writer regarding the applicant's strengths (and weaknesses, if applicable -- this can screw an applicant but some people will throw it in).
A final paragraph stating that the applicant is in the top __% of all the ___ with whom I have worked/interacted, or that the applicant is well suited for a career in medicine, or that the applicant is someone I would want as my own doctor. Optional: a phone number if the committee wants more information.