A great letter gives an admissions committee deeper insight into the person you are and the doctor you may become. It's your opportunity to let others make your case for you.
Hopefully, you've built relationships with individuals who would speak well on your behalf. Current professors and doctors with whom you work or volunteer are your best choices. Former professors and doctors with whom you've worked in the past are fine. (If you're applying to an osteopathic school, you must have a letter from a DO.)
A great letter comes from recommenders who can write about your specific traits and talents. Furnish them with a copy of your resume, a personal statement and any other material that will remind them what you've achieved. Also let them know which medical programs you're applying to and why.
Don't be anxious about approaching prospective recommendors, even if it's been awhile since you've worked with them. Professors and doctors are used to receiving this kind of request. Many will be happy (even flattered) to write a recommendation on your behalf.
If you apply directly from undergrad, you likely have access to pre-health or pre-medical advising, and your letters will be handled by that office. They will copy and send your recommendations to your list of schools. If you are a returning adult student, you may have to take care of all the requests and letters yourself.
Don't send more letters than a school asks for. There is one caveat to this rule: if you are placed on hold and work with someone during that time who might be able to contribute additional information, you can consider asking them to send a letter on your behalf. Anything more will be seen as extraneous.