Instant messaging is provided through one of a number of messaging applications (Windows/MSN Messenger, Yahoo! Messenger, Google Messenger, AOL Messenger, etc.). All parties to the discussion must be using the same messaging application. Unlike a chat room, discussion is constrained to a user's pre-defined contacts, with whom the user can communicate whenever the messaging software detects they are online.
Although most instant messaging is in text, it is becoming increasingly common for communication to be extended to voice (see 'VOIP') and picture (with the aid of a webcam).