MisCon 28 Writer's Workshop
Similar to a query letter a writer sends to an editor or agent, the cover letter should include a short paragraph about you and a paragraph about your project. Don't go overboard with this, just introduce who you are and what you are submitting. Let your writing speak for itself.
While the cover letter isn't necessary, it helps give the critiquing author an idea of who you are and what your stuff is all about, as well as some experience in writing professional cover letters. It can be as small as a few sentences, if you like (don't stress out about it), or as large as a few paragraphs.
If you submit a novel excerpt, you may choose to include a brief synopsis of the story. The point of the synopsis is to give a critiquing author, literary agent, or editor an idea of where the story is going beyond your submission. This is also a great exercise for any aspiring writer, since you will have to write one as soon as you are ready to submit to an agent or editor.
This does not count toward the total word count of your manuscript. It will not be included in the Anthology either, but will be given to the Critiquing Authors. Take a look at John E. Stith's example on the SFWA website. Just skip down to the green "Synopsis" heading to see how he does it. There are many options out there, so look around to find something which works for you. We tend to follow SFWA's lead, which is safe if you want to publish fantasy or science fiction.