From Mac Guides
iChat AV is an Instant Messaging application bundled with Mac OS X. In addition to text chat, it has support for both audio and video conferencing via an iSight for video or any audio source for audio conferencing. It is based on the AOL model, and allows fulltime, four-way video conferencing (from Mac OS X 10.4 Tiger), up to 10-way voice chat, and the ability to send pictures as part of the chat (rather than a separate file download).
iChat not only offers connection to AIM (AOL Instant Messaging), but also includes Bonjour messaging (Rendezvous in Panther and Jaguar). This allows iChat to automatically find other users with iChat Bonjour messaging enabled on your local network — no server is needed. iChat 3, included with Tiger, also added support for the Jabber protocol for connecting to Jabber services such as Google Talk.
Mac OS X Server 10.4 allows you to run your own Jabber server for your network or the internet. It uses Open Directory for user management and authentication, so your Mac OS X account name is the same as your email address (if it is configured as a mail server) which can be used as your Jabber account name in iChat.
New features in Leopard
A number of features were added to iChat in Mac OS X 10.5 (Leopard). These include multiple logins, visibility, animated icons, video recording, backdrops, Photo Booth effects, screen sharing, tabbed chats and "iChat Theater". iChat Theater allows the user to show slides to contacts.