I don't want to ask dumb questions and I also read the content on their site, but I'm still not sure if it's just a twitter alternative or more. And what about identica? Isn't it a similar service like app.net?
It's a twitter alternative that will (hopefully) not treat users as product.
It's also being thought about as a more open developer friendly platform (eg no constraints on clients because the service needs to show adverts).