Generally, IM users must know each other's username or screen name to initiate a session. Contact or buddy lists of frequent contacts can be created. 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, users must be online at the same time, although nearly all instant messaging platforms enable asynchronous interactions between online and offline users. If offline messaging is not supported, attempting to IM an unavailable user results in a notification that the transmission cannot be completed. In addition, the intended recipient must be willing to accept instant messages. Most IM clients can be configured to reject certain users.
When a message is received, the instant messaging application alerts the recipient with a window -- often a pop-up 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 notify the user with a distinctive sound, such as a chime or chirp. The user can also receive a visual notification, such as flashing the IM window or its taskbar icon when a message arrives.
In the past, IM clients were often based on proprietary protocols, requiring that users use the same communication software. However, the use of open standards is more common today, enabling multiplatform instant messaging systems, such as Pidgin and Trillian.
Another important shift in IM is the way in which it's accessed and delivered. These apps were long deployed as desktop clients that had to be downloaded and installed. Now, instant messaging is 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.
