What is a cover letter supposed to do? Well, it accompanies your story or poem, and it is often the first thing an editor looks at. Mind you, the editor actually wants to see the story or poem, so a cover letter should not be distracting. Also, the editor is so curious to read said story or poem that they can't wait, so the cover letter should be brief.
Here I have to defer to Hal Duncan, because he did one better than this post and actually tweeted a professional cover letter:
"Dear X, pls find enclosed "Y" (ZK words) as a submission for Yr Mag. Thx 4 yr time & consideration. The end"
Like this, please.
If you are not sure who will read your submission, write "Dear Editor." If there is more than one editor (you can find stuff like that out by, you know, looking at the guidelines and browsing the zine's site for a couple of minutes), write "Dear Editors." If you know who will look at the sub, it is perfectly acceptable to address them by name (Dear Ms./Mr. X).
Do not ever address an editor with "Dear Sir or Madam." Just don't.
If it's a cover letter for a poem, you can include line and/or word count (with most sf/f markets, it is acceptable to omit this part altogether when you submit poetry as they will pay a fixed amount in case of acceptance.)
There is no need to give any sort of summary or background info. Say you have a poem or story that heavily relies on the myths and folklore of some small island somewhere. Interesting, but no need to mention it. The piece should always work on its own, your storytelling skills still need to be solid, no matter where your muse hails from. The editor will in most cases be able to tell that there is folklore or myth in there anyway, and if they have a specific question, they will get back to you. If you feel the editor won't be able to tell that folkloristic aspects are important to your writing, it might be better to consider another market altogether. (Reminder: stories and poems are supposed to work on their own merit.)