If it's possible ;), we have even less info right now on multiplayer, then we do on singleplayer. By the way, I am NOT saying blizzard is behind or anything, I know the game isn't close to done yet. :) All info is subject to change.

The best way to multiplay, in my eyes and many others, is Battle.net. It is a program, which allows to to host, join, and chat. It's great, and used in Starcraft, Diablo 1, and Warcraft 2:BNE. It for sure will have some new features in it, cause we can always count on Blizzard to make multiplaying better. They are also doing this in Diablo 2. The whole multiplaying system for Warcraft 3 is in it's earliest stages right now though.