Use the cover letter to explain why you are interested in this particular company/organization/position, and to elaborate on your experience. That said, be careful not to simply restate what's already on your resume.

Definitely double check spelling a grammar. You don't want a simple mistake to trip you up.

(fyi, cover letter is two words)

keep it short and relevant. 2 or 3 very short paragraphs is all you need. Google cover letter samples and see how they are written and how you can tailor them to the jobs you are looking for.

HR folks or really anyone, what are some of your coverletter do's and don'ts? I ask because I always struggle with writing these.