A rejection letter is sent to job applicants who were unsuccessful in their bid for a position at your company. Rejection letters are usually sent after the closing date of the job applications at the stage that you invite successful candidates to interview. These are often form letters auto filled with the name and details of the applicants.
Rejection letters are not usually sent to candidates who you have interviewed. As you have met these people in person, a rejection letter can be seen as too cold and you may call the person instead to break the bad news to them yourself.