Definition: ICQ is a software system for instant messaging originally developed in 1996 at a small company named Mirabilis. ICQ allows any two registered people on the Internet to interact either "live" (like a person-to-person chat) or disconnected (like email).
Any Internet user may download ICQ and install the free client program. To register with the ICQ system, a person chooses their public username (handle) and private password. To use ICQ, a person simply logs in to the client program with that information. ICQ keep track of all the registered users logged in to the system at any given time. Therefore, people can keep buddy lists of their friends and be notified when these people log on to the system.
The ICQ client software works by connecting to an ICQ server on the Internet. When the program is launched, it initially finds and connects to a server using UDP port 4000. This allows clients to discover other online clients. For person-to-person communications, ICQ sets up and tears down point-to-point TCP connections as needed.
