Windows 8 Versions

Windows 8: What used to be the Home version. But missing the Media Center, which will only install on Windows 8 Pro.
Windows 8 Pro: What used to be the discontinued Ultimate version. In Windows 7 the distinction between Pro and Ultimate was the inclusion of Bitlocker and multi-language support. It really wasn't enough of a distinction to warrant two versions. Ultimate is gone.
Windows 8 Enterprise: Essentially for large corporations with site licenses. Features very similar to Pro.
Windows RT: It's really just half of Win8. It has the (no longer called) Metro app interface, but no desktop or ability to install Windows programs. It will only install apps from the Microsoft app store.

All apps can be reached from the apps tile in the interface. No URL is necessary.
All the promotional copies of Windows 8 died on January 31, 2013. We are back to regular pricing for everything.
Windows 8 Upgrade: The old Home version runs $70 - $80. Be sure to run the Upgrade Assistant before purchasing as it will let you know if your old machine will tolerate Win8.
Windows 8 Pro Upgrade: Runs $90 - $100. Again, run the Upgrade Assistant first.
Windows 8 Retail: About $120.
Windows 8 Pro Retail: About $150
Windows 8 Pro Pack: This is an upgrade from Windows 8 to Windows 8 Pro, enabling BitLocker, Joining a Domain, Windows Media Center and Client Hyper-V. Because of the expense of this upgrade, if home users think they might want Media Center, it pays to go straight to Pro even though it isn't for a business.