iChat is an instant message
client that comes with Mac OS X
iChat was first released as part of Mac OS X 10.2 Jaguar, where it was part of the default installation and even installed into the Dock as an AIM icon with a blue speech bubble. It worked with AOL Instant Messenger through a deal with AOL, where they were given access using the OSCAR protocol, meaning they were allowed official access to the network as opposed to the other third-party clients. It also allowed you to use a mac.com account as your screen name. It had features like beautiful speech bubbles as IMs (you could turn it off if you desired), and links to the OS X Address Book, where screen names would be linked to the person's real name. It also offered Rendezvous support (now called Bonjour) using the Zeroconf standard, allowing others on the LAN to IM each other and share files with drag-and-drop without any special network configuration. It also allowed you to save your IMs in log format and can run as a background service with iChat closed.
Later on, before Mac OS X 10.3 Panther was released, Apple announced "iChat AV," which allowed full-screen 30fps videoconferencing using firewire webcams and broadband. Apple began selling the iSight for this application. It was available as a limited time beta for Jaguar users and touted as a major selling point in Panther. It also allowed voice chat in an easy interface and the icon changed from a blue speech bubble with an exclamation point to a new camera icon. Bluetooth headsets were also touted as a feature.
Mac OS X 10.4 Tiger had a bunch of new iChat features, as well as a new icon of a blue speech bubble with camera icon. It allowed videoconferencing using the new h.264 codec, which gave a much clearer image, with up to 4 participants (as long as one of them had a fast enough computer) or voice chat with up to 10 users. It also featured Jabber support and allowed encrypted IMs using SSL certificates on a .Mac account or Tiger server. The Jabber support also allowed the use of Google Talk. iChat also allowed the use of USB webcams in 10.4.9.
Mac OS X 10.5 Leopard has a new list of features, such as special visual effects like those in Apple's Photo Booth application, as well as the ability to change the background of the video as a backdrop. It also promised tabbed browsing, video session recording, the ability to set your online status to invisible, the the ability to log into multiple AIM/.Mac accounts at once, live animated user icons, sharing slideshows, presentations, and movies over video chats, and screen sharing using OS X's VNC capabilities.