There is no "Retail" version of Windows 8. There is only the System Builder. If you buy System Builder for personal use, then it is effectively a "Retail" version because the Personal Use License applies.
You get the install media/DVD and a COA sticker/product code. Its a full version, meaning its not an upgrade version. You don't have to have a previously installed OS to use it.
It's the version of Windows you are expected to buy if you go to NewEgg and buy all of the parts to assemble your own computer. In this case, you wouldn't have a previous version of the OS to upgrade to on this new hardware, so you need to buy the full license.