Employers use resumes to determine whether or not they would like to contact a potential employee for recruitment. The resume does state your most basic information and your overall history but doesn't tell people much about you or how you approach issues. The cover letter letter gives you an opportunity to show how your knowledge and experience relate directly to the position you are applying for.
The cover letter is used as a quick "sound bite" of who are and why they should hire you. It should address two fundamental issues.
1.) How Your Knowledge and Skills Relate to the Position
2.) Why your the Best Candidate
Create a lead in paragraph that states why you are the best candidate for this position considering 1 and 2. The next paragraph (s) will give a little more detail on why that is the case. You will want to close with an action statement and request to contact.