Once you are done chatting on a server/network or you just want to leave.
This is done via the quit or exit command.
In some cases, both commands have the same effect: closing your IRC connection and terminating the IRC client.
/quit [message] /exit
Example: /quit Signing off. Goodnight!
In some clients, such as mIRC, the /quit command will only terminate the connection to the current server, the /exit command being required to close the client. irssi supports the /disconnect command for closing a connection without terminating the client.