Chats cover 1-on-1 Chats as well as private group Chats.  When you create a Chat, it will automatically be set to Private, regardless of the number of members invited.  Private Chats will always be available in the "Private" tab of "My Chats" irrespective of the amount of activity that takes place in the Chat.

Rooms cover group Chats that are able to be public and joinable by any non-blocked users. Join permissions may allow a Room admin to make a Room semi-private by restricting who can join, but a Room is always changeable back to public. When public, Any users may search for active Rooms using the "Live Rooms" section on the homepage. ("Discover"). Rooms are then either online or offline but Rooms in both modes will show up in the "Rooms" tab of "My Chats."

Group Private Chats and Rooms share many features and you may find functional similarities in both.  For example, the Admin Portal in both contain many similar options and functions, so please be sure to check help articles for both Private Chats and Rooms if you cannot find the help you are looking for.