It is meant to give high level information about your qualifications, but more importantly: it is your chance to showcase your PERSONALITY!
Show how who you are - makes you the best candidate for the position!
So what does that mean to you?
Make It Match: Use the same font, margins, and look/feel formatting from your resume.
Keep It Brief and Direct: Limit the cover letter to one page.
Be Unique: Don't copy and paste a general letter. Research the company to identify their goals/mission and show how you align to those goals! What do you bring that will help them meet those goals? Also how do you fit into the company's culture? Avoid cliche' expressions such as "self starter" or "go getter".
Expand on Your Resume: Do not repeat what is in your resume, instead go deeper to show your depth of expertise and experience. Competently speak to current industry trends and issues.
Show Yourself: Put your personality into your cover letter! It's your chance to introduce who YOU are!
PROOFREAD!!! Take a second and third look. Have someone else review the letter. Make sure you meet all submission requirements (subject line, attachments, etc.).
Stay away from generic, low impact wording like "responsible for", "provide", etc. Use specific action verbs with high impact!
Resources for High Impact Action Verbs: