What is the difference between a group chat room and a personal chat room?

Both types of chat room appear under Groups in the Chat panel, but they are different in the following ways:

• Group chat rooms appear under Official Groups and include anyone who is a member of a group and any non-members who want to chat with or monitor the room (if the group is public). Only admins or group owners can add or remove members from groups. 
• Personal chat rooms appear under Personal Groups and only include people invited by the room creator or by other participants. Anyone can create personal chat rooms, and they are not connected to groups. For more on how to create a personal chat room, see Create a chat room.