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Instant Messaging, often shortened to IM or IM'ing, is the exchange of near real-time messages through a stand-alone application or embedded software. Unlike chat rooms with many users engaging in multiple and overlapping conversations, IM sessions usually take place between two users in a private, back-and-forth style of communication.

One of the core features of many instant messenger clients is the ability to see whether a friend or co-worker is online and connected through the selected service -- a capability known as presence. As the technology has evolved, many IM clients have added support for exchanging more than just text-based messages, allowing actions like file transfers and image sharing within the IM session.
Instant messaging differs from email in the immediacy of the message exchange. IM also tends to be session-based, having a start and an end. Because IM is intended to mimic in-person conversations, individual messages are often brief. Email, on the other hand, usually reflects a longer-form, letter-writing style.

How instant messaging works

Generally, IM users must know each other's username or screen name to initiate an IM session or to add them to their contact list or buddy list. Once the intended recipient has been identified and selected, the sender opens an IM window to begin the session.
For IM'ing to work as intended, both users must be online at the same time, although nearly all instant messaging platforms now allow asynchronous interactions between online and offline users. If offline messaging is not supported, attempting to IM an unavailable user will result in a notification that the transmission cannot be completed. In addition, the intended recipient must be willing to accept instant messages, as it is possible to configure the IM client to reject certain users.
When an IM is received, it alerts the recipient with a window containing the incoming message. Or, depending on the user's settings, a window could indicate an IM has arrived along with a prompt to accept or reject it. Many IM clients also notify the user audibly with a distinctive sound, such as a chime or chirp. The user can also be notified visually by flashing the IM window or its taskbar icon when a message has arrived.
While IM clients were frequently based on proprietary protocols in the past -- requiring both users to use the same software in order to communicate -- the adoption of open standards has become more common. This has enabled the rise of multi-platform instant messengers, such as Pidgin and Trillian.
Another important shift in IM has been how it's accessed and delivered. Long deployed as a desktop client that had to be downloaded and installed, instant messaging is now more often found as a feature within another web- or cloud-based service -- such as Facebook, Gmail and Skype -- or as a mobile app, such as WhatsApp Messenger.

Features of instant messaging

The exchange of text has long been the chief function of instant messaging, but it is now one feature of many. The ability to insert images and emojis into messages is now standard in many clients, as are file transfers. Facebook Messenger even enables users to send money via IM. Numerous clients now support the escalation from IM to other modes of communication, such as group chat, voice calls or video conferencing.