Ahh... The beauty of the modular system. What the above posters have stated is correct. The stripped lower receiver is serial numbered and has to be registered. If you buy one online, it'll be shipped to an FFL and you have to pay the fee. After that, it's up to you. You can build an AR Pistol or Rifle. Want a 16" upper? Get one. 20"? Get one of those, too. Want one in .22LR? Yup. As long as you get a mil-spec lower and mil-spec components for the upper, it should all snap together.
If you're buying it for personal use, that's all you need to know. If you're buying it to build and sell, there might be some funky rules. Just make sure you keep your barrel length appropriate (ie, 16" or greater) unless you have a permit for a short-barreled rifle.