Columbia University's career resources page has this to say about cover letters:
A cover letter is a one-page business letter that you submit when applying to a job, along with your resume. As a piece of persuasive writing, your cover letter will aim to convey to the employer why you’re a great candidate for the role. Your goal is to convince the employer to interview you.
To this end, you should write your cover letter like Inigo Montoya presents himself to Count Rugen in Princess Bride.
To wit:
caveat lector