Last I knew about it, it was paid-subscription Twitter, essentially. Not totally sure what it is now. Honestly, I'm not sure they know what they are right now.
App.net is a network of social applications: App.net users can use their account to access hundreds of different applications available on a variety of platforms including Android, iOS, Mac, Linux and more.
Currently App.net users can access microblogging, broadcasting via push notification, groupchat, public or private chatrooms, photo sharing, journaling and more.
For developers, App.net offers a robust API and a variety of open source libraries and tools.
App.net is supported by offering paid services to the people that use it, rather than by selling advertising. Unlike other services, App.net users own their content. We believe that there is room in the market for a service that benefits financially from building things that people are willing to pay for.
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Its a network of networks which you can use to build your own network. Not being pedantic or sarcastic, that is what it says in the description. I guess the issue people have is understanding why this would be useful. I'm not sure why, but I'm probably not their target customer.
It's not really a network of networks, but a central repo for social networks. They can store all your contacts and photos (etc) there, and then you can choose to share your photos uploaded from one service with your contacts on another. It keeps your identity intact across the whole network no matter what app you're using.
