AOL Instant Messenger

The AOL Instant Messenger (AIM) is a free instant messaging computer
program, published by AOL, which uses the OSCAR instant messaging protocol.
The most recent software version is AIM 5.2, released in June of 2003. AOL
has described this technology as a means of "immediate cross-Internet
communication". [1]

It allows users to communicate instantly through text to their "buddies"
around the world, provided they have the AIM software. AIM has 100 million
users. It's easy to locate these users by visiting chatrooms that AOL has
set up solely for those purposes. Chat topics range from *NSYNC to current
affairs. AOL also has a member directory where AIM users can locate others
online who share their interests.

Since version 2.0, AIM has not only included person-to-person text
messaging, and chatroom messaging, but also the ability to share files
peer-to-peer with your buddies (unlike Napster, and other peer-to-peer
software, there is no directory of files, you only transfer files to one
another as one would in an email). Somewhere in the 4.x series, the AIM
client for Microsoft Windows added the ability to play games against one
another. Recent (4.3 and later) versions of the client software store your
contact information on AOL's servers, so you can talk to up to 160 of your
buddies from any computer with Internet access. Stand-alone official AIM
client software is available for free (but is not free software in the GNU
sense) for Microsoft Windows, Mac OS, Linux, Windows CE, and Palm OS.
However, some users stay on the 3.0 series because the software license
agreement for 4.0 and later clients includes a clause prohibiting the user
from ever using a third-party client program.

There is also a client called AIM Express implemented as an applet for the
Java platform that runs in your web browser. AIM Express does not have all
the features of stand-alone AIM clients (such as file transfer, buddy icons,
and away messages), but it still allows the basic functions of
person-to-person text messaging.

The standard protocol that AIM clients use to communicate is called OSCAR.
AIM Express uses another protocol called TOC. TOC has also been made
available to the public, in an attempt to throw a bone to third-party client
developers and lure them away from OSCAR. This scheme has not been
successful. Because AOL wants to be able to serve advertisements to users,
AOL has continually changed the details of the OSCAR protocol to keep
third-party clients such as Trillian from working properly. This has
resulted in a cat-and-mouse game between AOL and the client developers.