The difference between your resume and cover letter is that your resume should provide the reader with a better understanding of who you are. Your cover letter should connect the dots and show how your previous experiences translate to the job for which you are applying.
Developing the Body of the Cover Letter
Read the job description.
From the job description, select 2-3 key responsibilities or qualifications that you possess.
For each of the 2-3 responsibilities or qualities that you have picked, develop a specific example that represents how you have previously performed similar tasks or displayed similar qualities/skills. (This evidence can come from your previous work or academic experiences.)