is a popular advertisement-supported instant messaging client and protocol provided by Yahoo!. Yahoo! Messenger
is provided free of charge and can be downloaded and used with a generic "Yahoo! ID" which also allows access to other Yahoo! services, such as Yahoo! Mail, where users can be automatically notified when they receive new email. Yahoo! offers PC to PC telephone, file transfers, webcam hosting, text messaging service, and chat rooms in various categories.
In addition to instant messaging features similar to those offered by ICQ, it also offers (on Microsoft Windows) many unique features such as: IMVironments (customizing the look of Instant Message windows), address-book integration and Custom Status Messages. It was also the first major IM client to feature BUZZing and music-status. Another recently added feature is customized avatars.Source: Wikipedia