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 the sound someone makes when they were to whisper (eg. Pst over here) this is often mistaken as an acronym for Please send tell.

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

All four commands are functionally the same.