AIM (AOL Instant Messenger) is an instant messaging program that uses the OSCAR instant messaging protocol and the TOC protocol to allow registered users to communicate in real time. It was released by AOL in May of 1997.
Yahoo! Messenger is another popular instant messaging program provided by Yahoo!. It is used with a generic "Yahoo! ID" which also allows access to other Yahoo! services, and chat rooms in various categories. it was released on 21 June 1999. It is the first major IM client to feature BUZZing and music-status. Another recently added feature is customized avatars.
AOL Instant Messenger versus Yahoo Messenger comparison chart