How do I get started with World Of Warcraft?

I’ve played the single-player Warcraft III back in 2002 or 2003 when it came out. Now I want to see what all the hype about World of Warcraft is about. So how does it work?

I went to the local game store and they had more than one version. One was called "World of Warcraft" and the other was called "World of Warcraft", with a subtitle.

So which one is it? Or can I just get it online? What’s the difference between these?