First, are these articles essays or articles? If they are already written essays, you may be better off sending the whole article, but make sure you know what the specific magazine prefers. If the essays' merit is acheived via your writing quality and style, it might be hard to explain this in a query. However, most magazines (especially the bigger ones) prefer queries first. Editors recieve a lot of submissions and queries, and it takes a lot less time to read through a query than the whole article. Again, each magazine has its own preferences, and writers who adhere to them will gain bonus points with editors.
Pertaining to sending articles or queries to more than one magazine at a time: This is called simutaneous submissions. While every editor wishes that they were the only magazine you sent your query to, most understand that in order to make a living in this business freelancers rely heavily upon having the same idea published in multiple locations. It can take anywhere from a couple weeks to 6 months (or even a year) for a magazine to accept or reject your submission (and in some cases an editor may not send an answer either way). However, there are some magazines that specifically ask their contributors not to submit the same material to other mags. Generally, if you can't find anything that states this, it is considered acceptable to submit simutaneously. If you get an essay accepted, most writers will send the other editors a note stating which rights are still available. If the article idea (query) was accepted, it is common curtousy to tell the other editors.
My best suggestion is to pick up a copy of the Writers Market Guide 2010 (or use one at a library). This book will give you specifics for over 2,000 magazines.
Best of Luck,
KZ