A cover letter provides an introduction of the applicant to the prospective employer by describing qualifications. The information in the cover letter complements the information in the resume instead of just repeating the same information.
Companies with open positions often request cover letters with applications. The company may specify certain information to be included in the letter. A cover letter also works when prospecting for a job with a company who might not currently be hiring. The typical information includes the desired position, how the applicant heard about the job, relevant past experience and skills useful for the position. The cover letter is also used to indicate how the applicant plans to contact the potential employer to follow up.