A cover letter, as used in the job_hunting context is a document that is supposed to provide an introduction to the resume or curriculum vitae, which are the basic document employed when marketing one's own services to a potential employer. In the traditional way of doing things (when job hunters used 'hard copy' applications), the 'cover letters' was typically attached on top of the curriculum vitae or resume, thereby typically serving a cover in the literal sense of the word, to the Curriculum vitae or resume; and it is probably from this fact that it probably got its name 'cover letter'.
Objectively speaking, in that arrangement, the letters used to serve as a 'cover' _ that is, or the front page, to the whole job_hunting document, which apart from incorporating the curriculum vitae or resume, typically also tended to enclose certified copies of ones academic and professional credentials. The days when the letter used to literally cover the job_hunting documents might be long gone (as we mostly send applications, Curriculum vita or resumes and copies of certificates online as email attachments), but the cover letter has still not outlived its usefulness. The letter is the document that nowadays provides introduction to the whole job application attachment, 'and where it is all coming from. 'There are a number of formats that can be employed in drafting a cover letter whenever job hunting, depending on factors like the job that is being sought and the jobseeker's preferences.