Presence information 

  In computer and telecommunications networks, presence information is a status indicator that conveys ability and willingness of a potential communication partner—for example a user--to communicate. A user's client provides presence information (presence state) via a network connection to a presence service, which is stored in what constitutes his personal availability record (called a presentity) and can be made available for distribution to other users (called watchers) to convey his availability for communication. Presence information has wide application in many communication services and is one of the innovations driving the popularity of instant messaging or recent implementations of voice over IP clients.

Presentity

The term presentity is a combination of two words - "presence" and "entity". It basically refers to an entity that has presence information associated with it; information such as status, reachability, and willingness to communicate.
The term presentity is often used to refer to users who post and update their presence information through some kind of presence applications on their devices. In this case presence information describes availability and willingness of this user to communicate via set of communication services. For example, users of an instant messaging service (such as ICQ or MSN Messenger) are presentities and their presence information is their user status (online, offline, away, etc.). Presentity can also refer to a resource or role such as a conference room or help desk.
A presentity can also refer to a group of users, for example a collection of customer service agents in a call center. This presentity may be considered available if there is at least one agent ready to accept a call.