This command will allow you to send a private message to a specific user or send a message to an entire room. This command is exactly the same as PRIVMSG.
/msg Web How are you?
/msg #webchat hey guys, what's up?
Sending a msg to the room is exactly the same as speaking in the room. With some clients it is easier to specify the room in the command than to just speak in the room normally. Rooms that are not set +n will allow you to msg them even if you are not in them, but this is rarely useful. On the other hand, sending msgs to specific nicks is often useful. This is a good way to have private conversations or just send quick messages to friends.