When manufacturers create an Android device, they need to get it certified by Google (and probably pay a fee) to use the Android Market, and other apps like Gmail and Maps. But some manufacturers decide not to, and create their own app store. To get the Market on there, you’d have to side load the apk, and then install it on manually. Once this was completed, you still couldn’t install apps from the Android Market website straight to the device. This changes now, as the website is currently supporting unofficial devices.
Anyone with a cheap eBay tablet or even Kindle Fire can now have hopes of getting the full Google experience. I noticed this with my HP TouchPad (unofficially running Cyanogen Mod 7), and I can confirm that it works.