It appears there are four main worlds of magazine publishing, with little overlap.
The first is the literary world, a vast space that might encompass as many as 1,200 literary magazines. That’s according to Poets & Writers (external link). These are mostly products of University English Language and Literature Departments. The pay is usually in copies. And while competition is still keen, your chance of getting a personal essay published here is far greater than in the New Yorker or the Atlantic Monthly.
The second world is that of the commercial press, with most articles focusing on nonfiction. This is the territory of Wired, National Geographic, Esquire, and Rolling Stone. Everything you see on a newsstand. Along with smaller publications that cater to speciality interests such as hobbies and travel. I’d say the competition is tougher here than in the literary world, perhaps because you will get paid for your work, although less and less every year.
The third world is that of Academia, or any writing that requires a degree. University Presses focus on catering to professors needing to publish for tenure and job security. There is little chance a non-academic would be accepted into this world, no matter how keen the interest or original the idea.
The fourth world, and an outlier, is the institutional, corporate, or government publication. I once wrote a 6,000 word article for Telektronikk, a publication of Telenor, Norway’s government run telephone system. And right now I have an article in process for a California state agency. Neither of these magazines are found in any bookstore, nor for that matter, are inflight magazines like those published by Southwest Airlines or TWA. Throw in member only magazines published by automobile clubs and the like and you’ll have quite a list of magazines that solicit articles but are not what one first thinks of when preparing a query letter. Oh, and don’t forget trade magazines. Visit the magazine room of any large University and you’ll find plenty of titles that might carry your work.