Windows 8 Pro is the operating system to which Microsoft wants you to upgrade. You will of course be able to buy a Windows 8 Pro version separately if you are installing it clean on a new computer.
For all intents and purposes, this is the complete Windows 8 experience. It gives you everything Microsoft has put together for the new operating system, including the Modern UI look that you get in Windows RT.
This version of Windows comes with the facility to add on Windows Media Centre, Microsoft's computer-based media management software. The Media Centre can sync with a TV tuner and lets you record and watch live TV. It can also be used to manage and play back music and movies.
Other than that, Windows 8 Pro doesn't add much over the standard edition that's of use to the average computer user, other than the ability to boot from a VHD and join a Windows domain.
You can install Metro-style apps to Windows 8 Pro from outside the Windows Store, but quite how that process will manifest itself and whether it will be something important to do remains unclear for now. We certainly wouldn't lose any sleep over it.