Origin messaging provides end to end encrypted communication between two users, using ethereum addresses as their identity.
A room is everything exchanged between two users. Each room uses its own derived keys. This allows a user to give an arbitrator access to only the conversations between themselves and their counterparty, without having to open up other conversations.
At a data level, a room is a list of both the messages from either party and the public keys used between them.
Currently Origin Messaging hosts a service to exchange encrypted room messages, however more distributed message exchanges are possible.