A cover letter is a one-page formal letter to the company that summarizes why you are interested in and qualified for the job you are applying for. You should be writing a cover letter for a position you are hoping to obtain either in the near future or after you graduate. Their are five different components: the header, introductory paragraph, the body, closing paragraph, and the farewell.
The link below is a PDF of the Entry Level Resume Guide from DePaul University Career Center. This guide will assist you in writing your resume.