Usage: PART <#channel> [<message>]
The part command has you leave a channel. The other people in the channel will see the message if you send one. The example below would make you leave the channel #webmaster.
/part #movies see you today
Note: the server configuration may surpress the display of the part message. This is because part messages, just like QUIT messages, can easily be abused to spam or annoy other users.