Instant messaging, often shortened to simply "IM" or "IMing," is the exchange of text messages through a a software application in real-time. Generally included in the IM software is the ability to easily see whether a chosen friend, co-worker or "buddy" is online and connected through the selected service. Instant messaging differs from ordinary e-mail in the immediacy of the message exchange and also makes a continued exchange simpler than sending e-mail back and forth. Most exchanges are text-only, though popular services, such as AOL, MSN Messenger, Yahoo! Messenger and Apple's iChat now allow voice messaging, file sharing and even video chat when both users have cameras.
For IMing to work, both users must be online at the same time and the intended recipient must be willing to accept instant messages, as it is possible to configure the IM client to reject chat sessions. An attempt to send an IM to someone who is not online, or who is not willing to accept IMs, will result in notification that the transmission cannot be completed. If the online software is set to accept IMs, it alerts the recipient with a distinctive sound, a window that indicates that an IM has arrived and allowing the recipient to accept or reject it, or a window containing the incoming message.
In the past, both users had to be using the same software. Many current clients, however, allow interoperability between networks, including Microsoft's recently developed Live Messenger.
Under most conditions, IMing is truly "instant." Even during peak Internet usage periods, the delay is rarely more than a second or two. It is possible for two or more people to have a real-time online conversation by IMing each other back and forth.
Once in a while, a person might receive an IM from someone while already engaged in a chat with someone else, and decide to carry on IM chats with both people independently and concurrently. This requires mental alertness to avoid the embarrassment of sending one IM companion a message intended for the other, though younger "netizens" often pull off this level of multi-tasking as a matter of daily practice.
Developments in mobile cellular phone technology now allow handset users to always be available via IM, through such popular devices as the T-Mobile Sidekick II or other smartphones.