A "Cover Letter" is a persuasive document that introduces you to the employer and highlights why you'd be a good fit for a specific position. In nearly all cases, a cover letter should accompany every application and resume that you send out!
Cover letters are most effective when customized to each position for which you are submitting an application. If a specific position is not open, you can write a cover letter to a specific organization to inquire about the potential of filling a need. Take the time to research the organization and connect your skills and experiences to the employer's needs and how you can contribute.
In your cover letter, address why you are interested in the organization or the specific position and use key words from the job posting. Rather than repeating information on your resume, use your cover letter as an opportunity to draw the employer’s attention to a specific example of how you are qualified or summarizing and enhancing personality traits that make you a desirable employee. Remember that your cover letter is an opportunity to demonstrate your writing abilities as well, so pay attention to grammar and writing style.