Tells are a way for people who have fully introduced themselves to each other to be able to communicate with each other telepathically. Those who have been elevated to the status of hegemony can send and receive tells to anyone, regardless of introductions. Just because a tell is sent doesn't mean the person is awake, or has forgotten you. Be sure to check your friendship band to know for sure if your friend is awake.
To send a tell to another player, simply type "tell <username> <message>". Emotes can be used in tells as well through the ";". Please note that tells sent to someone in the same room as you will not be received.
The tell command has also been globally aliased to the " symbol.