Whisper

From WoWWiki

Did you mean the NPC named "Whisper"?

A whisper, or tell, is a private message sent from one player to another, often denoted by the color pink in the chat window.

PST

PST is an acronym for "Please Send Tell." This is a request that any responders use one of the following commands to reply:

• /whisper <player_name> <message>
• /w <player_name> <message>
• /tell <player_name> <message>
• /t <player_name> <message>

All four commands are functionally the same.