It's just a Twitter clone, that charges users for membership, instead of monetizing them through ads or selling their data. A lot of people think that will lead to a better service, so it got a lot of hype recently.
The twitter clone is a proof of concept. A better description would be that its a paid social graph designed to be used by third party apps. Just like Facebook apps or apps that use twitter to sign on, app.net can be used as that identity- the difference being that app.net is promising not to put arbitrary restrictions on how it is used.
