Had a look at the commercial. As we already know, you'll be able to find Pokemon and items in the Dream World and transfer them to your game. It appears you meet Pokemon through a graphical interface like that of the games, and the Pokemon appear as overworld sprites.
You also get a "home", where you can grow berries. You can buy and arrange furniture inside your home, and visit your friends' homes to "play". (No elaboration on the point of doing this)
There's also a Global Battle Union, or GBU, whose features CloneCharizard previously explained.
You'll also be able to send private messages to your friends. Oh, and you can download "covers" for your Pokedex.
That's about it!