Whisper is an action that can be used in chat to send a private message to a user.
How to whisper
To whisper, enter a forward slap then the players name (e.g. /pooh713) or, in the chat window, click on the player's name and select Whisper.
Names with Spaces
On newer servers if you click on the persons name that you want to whisper to, then click whisper, it now adds in the ' ' on the ends of the names automatically if there is a space in them. If you are going to be whispering a few times, you could copy the whisper text to save time.