A cover letter is simply that. A letter that you write and attach, send along with, your CV that both literally and figuratively "covers" your resume. Literally, the letter covers your resume/CV, because it is placed in front of/on top of the CV. Figuratively the cover letter "covers" what you want the prospective employer to know about you, your qualfications and experience, what you are looking for in a job and how your background and experience qualify you for the position they are looking to fill.

My cover letters pretty much address:

1. Why I'm writing the letter.
2. What I can do for you.
3. Why I want to work for you.

There are many examples of cover letters on the web. There are also several books available on writing cover letters and how to write the best one for each situation. A cover letter is very important in making a first impression.

I highly recommend that you go here: http://ask4essay.com/how-to-write-a-cover-...or-a-first-job/ for cover-letter advice.