Rooms are an publicly accessible Scuttlebutt peer, but unlike pubs they don't host feed data from other peers. The main feature rooms provide is a way for peers to find each other and establish tunnelled connections among themselves as if they were on the same network. They also provide more moderation tools than a pub with multiple user-levels and different privacy modes.
Rooms differ from pubs because even though they're a Scuttlebutt peer, they are not meant to aid replication by diminishing the number of hops between peers. Instead, they provide a simple way for peers to find and connect to each other even though these peers might not have publicly accessible connections, and thus no follow messages are used in the rooms workflow.