It unfortunately requires Windows, IE, AIM, and Realplayer to use... but read up on it... it's worth it.
Basically, the game is some sort of mystery/spy/conspiracy theory game. The difference between it and a "normal" one though is the level of immersion - you don't just play the game, you really interact with it. It will IM you (over AIM), call you (if you choose to give it your phone number), send you faxes (again if you give it a fax number) and give you hints online. You look through web pages for clues, chat with the people and bots on AIM, get emails and follow the clues in them, etc. You also get video and voice messages on the site to listen to for clues, etc.
I don't know how good the full game will be, but the demo so far is really interesting. Part of that is just because the concept is so different from anything I've seen - I love the fact that my phone rang tonight and it was the GAME calling me. It's like being IN the game. For those with lower thresholds for that sort of thing, it can do things like announce that it's the game calling before getting in character, or not call at all (or call at specified hours only).
To add to the mix you're stuck in a pool with other players, who are your allies - they're going through the game at the same time, with the same basic level of knowledge, and you can contact them and chat with them. They're mixed in the same list as the bots and game characters, though, so at first when I started talking with one I thought it was a game character. :)
There are a lot of touches like that, that really act to draw you in. Yeah, it's obvious once you hear the message that it's a game message, and same with the emails and such, but it's still really cool to be involved in such a direct fashion.
I'm looking forward to the rest of the demo. We'll see how it goes. :)