This article is from the misc.writing Writing FAQ, by Wendy Chatley Green email@example.com with numerous contributions by others.
A cover letter introduces you and your work to an
editor. For most short fiction, a cover letter is optional;
many magazines don't really want one. For non-fiction, it
is almost mandatory. Of course, if you used a query letter
(see that section) to get the writing assignment, then a
cover letter may not be necessary; the editor already knows
what to expect from you.
A basic cover letter is:
[usual date and header]
Dear Editor (Use the correct name!)
Enclosed is my article on Usenet cookbooks--10,000 words as
you requested in your letter dated 5 January, 2000.
[usual closing and signature]
Do not use a cover letter to "sell" your story; if
an editor requested it, you've already "sold" it. At this
point, your writing must do the rest.