Using a new technology usually carries some advantages and disadvantages. The advantages of multicast are -I think- clear. The main disadvantage is that hundreds of hosts and, specially, routers don't support it yet. As a consequence, people who started working on multicast, bought new equipment, modified their operating systems, and built multicast islands in their local places. Then they discovered that it was difficult to communicate with people doing similar things because if only one of the routers between them didn't support multicast there was nothing to do...
The solution was clear: they decided to build a virtual multicast network in the top of the Internet. That is: sites with multicast routers between them could communicate directly. But sites joined across unicast routers would send their island's multicast traffic encapsulated in unicast packets to other multicast islands. Routers in the middle would not have problems, as they would be dealing with unicast traffic. Finally, in the receiving site, traffic would be de-encapsulated, and sent to the island in the original multicast way. Two ends converting from multicast to unicast, and then again to multicast define what is called a multicast tunnel.
The MBone or Multicast Backbone is that virtual multicast network based on multicast islands connected by multicast tunnels.
Several activities take place in the MBone daily, but it deserves to be remarked the profusion of tele-conferences with real time audio and video taking place across the whole Internet. As an example, it was recently transmitted (live) the talk Linus Torvalds gave to the Silicon Valley Linux Users Group.
For more information on the MBone, see:
http://www.mediadesign.co.at/newmedia/more/mbone-faq.html