IM is a communications service in which short messages appear in pop-up screens as soon as they are received, thereby commanding the recipient’s immediate attention. Most IM services offer presence information that indicates if the user is online and available to send and receive messages. These services also provide “buddy lists” that are groups of people who have been selected by the user for frequent access, as well as group-based chat services. Enterprise IM provides real-time message passing within private and public networks.