The term, Short Selling seems to be the most widely used, but they both have the same explanation.
I thought Dys' explanation was clear, but I wanted to get a link, so here is what I thought was easy to understand. I also understand why it cannot be regulated: individuals are spending their money on a product. That isn't illegal even when large corporations do it in a big way.
Here is a link with one paragraph that provides a clear example:
Selling Short Stock
http://addtrader.wordpress.com/2008/01/12/how-to-explain-short-sell
ing-to-a-non-trader-like-your-mother/
A share of stock is standardized, just like a book, meaning each
share of IBM is the same, just like every copy of Harry Potter and the
Deathly Hallows, on the shelf at the local bookstore is the same. So
let’s say your best friend just bought a brand new copy of Harry
Potter, and the moment he brought it home you asked him to borrow
it. Now that you have borrowed it from him, you owe him back one
copy of Harry Potter. So you take the copy he loaned you, and you
sell it. Maybe you sell it on Ebay, maybe at a garage sale, maybe to
an individual, it doesn’t really matter. You get $25.00 for the book
when you sell it. Now you still owe your friend one copy of Harry
Potter, so you order a copy on-line, at a discount book wholesaler.
That copy only costs you $19.95. The book arrives, and you return it
to your friend, and keep the difference between what you sold it for
($25.00), and what you bought it back for ($19.95). You have thus
made money by selling something you did not own, and then buying it
back (and replacing it) for less that you sold it for.