The Presence subsytem provides the ability to tell who is available for collaboration and what their current status is.
The entrance point is the PresenceService. User Presence tracks the availability and state of Domain Users within the System. Users are generally available or not if they have at least one session that is connected. Each user in the system can set presence state. Presence state is global for each user in that the state is shared across all sessions.
See the developer guide for additional background.