Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Instant messaging, or IM, provides a way to send short messages to other people on the Internet, usually in real time, although delayed delivery is supported. Instant messaging could be compared to a combination of e-mail and Internet chat. Like e-mail, instant messaging supports message exchange between people; but, unlike e-mail, instant messaging is designed for two-way live chats. IM bridges the gap between voice calls and e-mail. What IM does not normally do is archive messages like e-mail programs. Business users may need to look for products that log conversations for later referral or for legal purposes.

Chat and instant messaging are forms of synchronous communications. Like a voice telephone call, a chat or instant messaging session is live and each user responds to the other in real time. But while voice calls require the immediate attention of the parties involved, IM gives people a chance (if only momentary) to think out responses. In contrast, discussion forums and electronic mail are asynchronous communications. Some amount of time may pass before a person responds to a message. A discussion forum is a non-real-time chat room where groups of people exchange messages in an asynchronous style. A message sits in a message queue for other people to read and respond to at any time, or until the message falls out of the queue.

Important aspects of IM are presence monitoring (PM) and what AOL calls "buddy lists" or just lists. A list is a group of people that you want to exchange instant messages with. A presence monitoring server keeps track of when people log on and notifies you. Likewise, the presence server keeps track of when you log on and notifies other people. You can specify exactly who in your list should know you are currently online. For example, you can specify that fellow employees only know you are online from 8:00 to 5:00.