All valid answers. The d20 rules are the rules that comprise the D&D game. In other words, it's like saying cooking is a hobby, and recipes are the rules by which it is governed. Well, something like that. D20 is name of the recipes, or the rules by which the game is played.
D20 is also freely available to 3rd party publishers so that they can provide products that work with the D&D game, which ends up expanding the amount products that are available. Wizards of the Coast produces D&D products, but there are literally hundreds of 3rd party (mostly small, many of them individuals) that produce add-on products that are compatible with the D&D game.
To start, all you really need are some dice, and a Player's Handbook. If you're exclusively playing online, probably all you really need is a Player's Handbook. Even that is debatable, since all you *really* need is the System's Reference Document, which is available at
http://www.opengamingfoundation.org
But, that's *just* the rules, and has no descriptive text. So, it's like saying "1 cup sugar, 2 cups flour" without any real description of what it is you're making. So, I wouldn't recommend that. It's there because 3rd party publishers need to be able to know what they can and can't reference from the rules when making their products. Some things like proper names, and copyrights are not available.
You can buy a Player's Handbook at any game/hobby store (check the yellow pages under Games to find a local FLGS - Friendly Local Game Store) or just find one at Amazon.com.
Hope that didn't over-complicate things.