Private messaging and chat rooms
How they work and what they're called
Chat rooms can be between any group of people from 1 to 200. You can set permissions for different users to read, write, or moderate the room -- or you can set the room to "public", and let any user do what they like, or even people not on Pnut see the room.
Private messaging is a specialized kind of chat room, built just for you and a set of other users. The permissions are strictly set to you and the set of users who the room is made for. Sometimes it is shortened to "PM", "direct messaging", or just "messaging".
Sometimes clients/apps or users will refer to chat rooms as "messaging", or "channels", "chat rooms", or "Patter rooms" (an older term for chat rooms based on the Patter app).