The only difference is, recommendation letters are almost always specifically requested to be written about someone, and are therefore addressed to a particular requester.
The main rules to writing a recommendation letter is almost same to reference letter (read writing a reference letter). Just when writing the salutation, do not use “To whom it may concern”, or “Dear Sir” or “Dear Ma’am”.
A recommendation letter must be always addressed to an individual.
Acceptable salutations are “Dear Mr. Someone“, “Dear Madam Someone” etc.
Sample recommendation letters are below: