A recommendation letter or letter of recommendation, also known as a letter of reference, reference letter or simply reference, is a document in which the writer assesses the qualities, characteristics, and capabilities of the person being recommended in terms of that individual’s ability to perform a particular task or function. Letters of recommendation are typically related to employment (such a letter may also be called an employment reference or job reference), admission to institutions of higher education, or scholarship eligibility. Recommendation letters are usually specifically requested to be written about someone, and are therefore addressed to a particular requester (such as a new employer, university admissions officer, etc.), although they may also be issued to the person being recommended without specifying an addressee.
Don’t exaggerate -- show where and how they can improve. Don’t put the candidate on a pedestal. Not only does it not look plausible, it will also set expectations for them that will be next to impossible to meet. If they have an Achille’s heel, don’t exaggerate it, but don’t hide it, either.