Letter Of Recommendation
The term refers to a type of letter written by a person who can support your skills, abilities, competencies, and work ethics in front of a potential employer. The person writing the letter of recommendation is called a referee and can be a former co-worker, a former teacher or even a former employer.
The letter of recommendation is very useful whenever applying for graduate school or for a job, even if some may say in the last case it can be actually useless, because anyone giving you a recommendation in this regard will never say something bad about you. Nevertheless, it is better to be backed up by good references than having no reference at all.