You can send private messages to other users that only the two of you can see.
Starting a Conversation
/query <nickname>
This opens a PM tab. Alternatively, send a single message:
/msg <nickname> <message>
Using the User List
Click a nickname in the conversations sidebar user list and select Query to open a PM tab. You can also double-click a nickname to open a PM directly.
For a lightweight message that does not open a PM tab on the recipient's side, choose Send Notice from a nickname context menu or use
/notice.