The standard letter goes something like this:
Dear Committee on Admissions,
I am pleased to write a letter of recommendation for John Jones, an applicant to your program. writing "your program" leaves it open to use the letter for med school or something else.
The letter then goes on to say how long the writer has known the applicant and in what context (as a patient, neighbor, friend or classmate of the writer's child, as an employee, volunteer, as student in the classroom or laboratory, etc). If the relationship changed over time, that is described (e.g. you were a neighbor who went on to volunteer in the office for a year and was later shadowed the doctor in the OR).
Some writers will describe themselves, what they do and where/how they interacted with the applicant. This might include the type of practice, number of others in the practice, hosptial affiliations, and activies undertaken including procedures or surgeries. The writer might go on to describe the activites undertaken with the shadow (procedures, interpretation of diagnostic imaging or tests, surgeries, hospital rounds, outpatient clinic, deliveries, etc)
The writer then describes what he has observed (characteristics of the applicant) and his opinion of the applicant's maturity, character, intelligence, etc.
The closing paragraph is generally something to the effect that the writer recommends the applicant and how great the applicant is compared with others the writer has interacted with or that the applicant is as good as medical students or residents with whom the writer has worked.