A letter of recommendation is a letter where the author details and vouches for the capabilities, character traits, and overall quality of the person being recommended. This can be from a professional, employment, academic, or personal perspective.
These letters are also called a reference letter, recommendation letter, letter of reference, or just reference. They are most commonly used in the hiring process as part of the employment verification step where they are called an employment reference or job reference.
source : betterteam.com