It's basically a mesh structure, and users host the content they care about. Content that's valued will propagate, and junk will die off.
Each user has a PGP key pair, with peer discovery through distributed hash tables, like BitTorrent or Tox. When you add a contact's public key, you form a connection with that user.
So each user is a node in the mesh, and files, forums, and email style messages are shared peer to peer. The incentive to run a node is the use-value of the network at this point. I'd like to see further partnership between RetroShare and BitShares eventually, but using it is a good place to start with that.