Users can send eachother messages in real-time, regardless of the Swarm-enabled game or application they're using.
APIs are available so that you can customize the look and feel of Real-Time Messages between your users.
Just like e-mail, users can leave messages for eachother even if someone is offline.
If a user receives a message when offline, the message will be available in their inbox the next time they login.
Swarm handles all of this for you.