A cover letter acts as an introduction. It helps set you apart from other applicants by encouraging the employer to take a better look at your resume and/or application by explaining your suitability for the desired position. Send a customized cover letter with each resume you send out. Always send the original, not a photocopy. Your cover letter may make the difference between obtaining a job interview and having your resume ignored. Even if an employer doesn't request a cover letter, it's helpful to send one. Keep in mind, a cover letter should complement, not duplicate your resume. Expand on your resume and highlight your background as it relates to the job.
What information should you include?
1st paragraph: Briefly state the specific position you are applying for.
2nd paragraph: Restate your interest in the position with the company. State why you want to work for the company and why their organization should be interested in you. Emphasize your strengths, skills, experience, education etc… relating to the position.
3rd paragraph: Thank the employer for their consideration for the position and express an interest in an interview.