What is a Market Guide? It's your best friend, pal. But it is a little bit like a dachshund. While it may seem friendly, it's very loyal to its master. Pet, but don't tease.
         So, you have created the Great American - Something. Now, what are you planning to do with your work? If you want to send it somewhere, you really ought to know what sort of welcome it'll receive. Many budding authors go to the Yellow Pages (or DEX) for their locality and look up "Publishers." Don't bother. Most of them, particularly in small cities, publish newspapers, fish-wrappers and calendars. Not the sort of thing that you had in mind, probably.
         Aha! There are guides. If you're really getting into this business, there's no rule against actually buying the guidebooks. They're called things like "Writer's Digest" and "Writer's Market" and suchlike. But, wait - many of these annual and periodic guides can be found in even the smallest-town libraries. Sure, the ones in the library are out of date, but things change slowly in this business. By the time any hard-cover book reaches the shelf, whether market guide, how-to or textbook, so much time has elapsed since it was created that it's several years out of date. Still, they do have lists of publishers, along with detailed information on what is needed, what is paid, and how to submit your work. In addition, they often have helpful information on manuscript protocol and the like.
        Of course, your best guide for up-to-very-up-to-date is our pal, Calliope. As it is for almost everything else pertaining to the art and angst of writing. Write that down!
        Okay, pick up your market guide, whatever the source. (This is where fluency in italicized Urdu is handy.) First, does the publisher usually buy the sort of thing you've created? There are general categories, such as travel, and regional, and so on. In fiction you may find reference to F/SF/H, for instance. No, that doesn't mean "fifty/seventy-five/hundred" but Fantasy, Science Fiction and Horror. You may also see SS or S/S which usually refers to Sword and Sorcery. There are many, many other genres, of course, and many magazines accept all genres. The important thing to remember is that a Horror story sent to a strictly Science Fiction or Fantasy market will die right there in that bin under the reading editor's desk. Save your postage and read the guide first.
        Now, let's say you think you have just what the publisher seems to want. What then? Well, more Otter Confucian. You see things like "FNASR" or "Serial Rights" or "One-Time Rights" or "Anthology," etc. These all refer to the way in which your work will be used, if it's accepted. Commonly, you are selling FNASR (First North American Serial Rights,) which means that the buyer has the right to publish it anywhere on this continent with the agreement that you won't peddle it to another market on the same continent. And his "Serial" rights are just that - he can use it as often as he wants. "One-Time" rights provide a different deal. After the buyer squeezes the first profits out of the story, you are free to sell it elsewhere. For most writers, these distinctions may be unimportant. But I did run into a problem with an FNASR deal when I also sold the story later in India, because the publisher's representative was in New York. We worked it out, but it cost a bit of time and frustration.
        Some publishers also sell a "Best-Of" anthology now and then, usually yearly. If you agree to Anthology rights, he can do that without asking you. He probably will ask, anyway, which is his way of announcing that it will be available and you ought to buy a few copies to impress your kinth. [STET]
        If you're not sure what rights you're selling, ask the nice publisher in your query letter. (Slick segue?)

If you're approaching a new publisher (for publishing issues, that is, not one-night social arrangements) it is wise to send a query letter. As simply as possible, describe what sort of work you have in mind and ask if it would be appropriate. Again, you save postage and the editor is saved the trouble of chucking your work, UNREAD, down the Memory Hole.  
        Don't tell the man that what you have is just what his poor, little magazine needs. He might already know. And don't tell him how great! you are. Let your work show him. Use the same approach you would in writing a cover letter - be respectful, be humble and be brief.
        If a publisher says he will consider "simultaneous" submissions he means that you can go on and submit your work elsewhere at the same time you send it to him. There's a trap, there, though. He might give the nod to simultaneous submissions and you might just send the work elsewhere, and then, later, find that you have committed Cardinal Sin Number One. Yes, your work is accepted by the second publisher after the first one has accepted it and inserted it into the paste-up for the next issue.
        Ka-boom! Now you must meekly withdraw the thing and hope that you aren't making too big a problem for either publisher. They're like elephants - they remember and they hurt when they stomp on you. I have found that it's less bloody to have more than one piece of work to send around so I'm not tempted to submit to two (or more) markets and risk becoming a pariah. Remember - the publisher needs writers, but not you exclusively. You need him. Be nice.
        Yes, I have gotten myself into this situation - twice. Once was because I was truly greedy and wanted to get as much work into print at once as I could. The other time was a genuine mistake. The editors' reaction to both situations was the same. And not too comfortable. Be ye aware!
        What else? Oh, some publishers give you a hint as to what sort of prose they prefer. That's good, up to a point. But it's almost a cliché that publishers of Science Fiction say they want stuff like H. P. Lovecraft used to write. Well, sure, except that Lovecraft would sometimes take three pages to set the scene for his superb stuff, which is not going to fit into the usual quick-bite 2,500 word limits of many small press magazines. Still, it does sound nice.  
        One bit of information I find most important for new writers is the estimated response time. This is a rough measure of how long you remain skewered on your tenterhooks while the magazine's one or more levels of decision-makers sees your work and finally renders a verdict. It's well to just stop thinking about the derned thing after you send it off. If you wait by the mailbox every day for an answer, you won't get much writing done. And writing is why we're here!