Cover Letter

When I went through TAP before separating from the Air Force they taught me all kinds of great things about resumes, job interviews, etc.

However they didn't teach me about cover letters.

What is a cover letter? How do I make one?

I've looked at a few examples from Google, but I don't see the point. They basically seem to underline some qualifications you mention on the resume.

It's redundant. Please help.