A cover letter is sent along with a resume to provide additional information and highlight how you (the candidate) are relevant and fit a particular need of the recipient. It typical serves as an introduction, establishes for the reader what position (or assistance) you are seeking and hopefully entices them to read your resume. It is generally no more than one page in length. The letter should highlight your relevant skills and experiences in regards to the position that you are seeking. It is also your opportunity to present qualifications that may not be on your resume that match the needs of a position.
Traditionally, a cover letter was written as a formal business letter with the resume as an enclosure. E-mail has changed the way cover letters are used and delivered. When submitting a resume via email, the body of the e-mail could serve as the cover letter. Brevity is essential in an e-mail. Alternatively the cover letter and resume may be submitted as one file (a .pdf file will ensure that format is not compromised) included as an attachment to an e-mail.