Wc3:WoW will use a special third-party program called "WarSoc" which will enable Wc3:WoW to be a multiplayer game. People will be able to join the game even after it has been started and thus allowing possibly hundreds of people into one server, making this into an MMORPG. I'd really rather not explain the technicalities of how it works, what makes it possible, etc. So you'll have to ask MindWorX: He's the creator of the program.
Will Wc3:WoW or the third-party program cost any money? No. At least Wc3:WoW won't cost you a cent, that I know. Whether MindWorX decides to add a small fee on his program or not is his decision.
So here's the plan:
Wc3:WoW will be worked on as a Single-Player campaign, but acting as if it were an MMORPG. The single-player version is simply just a layout of how the MMO version will be, but offline and incapable of even lanning!
We're currently working on the Single-Player version to prepare the campaign for further MMORPG development. Once WarSoc is finally finished being made and is debugged, Wc3:WoW will be supported by it. Will we stop making the Single-Player version? Most likely not! A lot of people told me they want the Single-Player version more because they are strongly in favor of the smart computer AI of the Player-Bots and once the AI is fully developed, the bots will be doing things not even I could explain how they were smart enough to do... though this will take a very long time to make and it isn't our primary goal at this very moment.
Once we've finally gotten Wc3:WoW up and running, we'll being working on its stability and defiantely making Wc3:WoW a lot more like WoW, including the GUI.
~Craka_J