Chat is a suite of applications that handle "instant messaging"-style communication between ships.
A chat is a named collection of messages created by and hosted in a ship's chat-store, usually represented as
~ship-name/circle-name. Most of Chat revolves around doing things with these chats.
Chats are subscribed to by some number of ships which are permitted to post and retrieve messages from the chat.
There are four "kinds" of chats, that use different permission modes:
channel is a chat that is publicly readable and writable, with a blacklist for blocking.
village is an invite-only chat.
journal is publicly readable and privately writable, with a whitelist for authors.
mailbox is readable by its owner and publicly writable, with a blacklist for blocking.