mIRC - highly configurable IRC client
IRC (Internet Relay Chat) is a virtual meeting place where people from all over the world can meet and talk; you'll find the whole diversity of human interests, ideas, and issues here, and you'll be able to participate in group discussions on one of the many thousands of IRC channels, on hundreds of IRC networks, or just talk in private to family or friends, wherever they are in the world.
To use IRC you need a small application like mIRC, an IRC client for Windows that was written by Khaled Mardam-Bey. mIRC is a friendly IRC client that is well equipped with options and tools.
There is no restriction to the number of people that can participate in a given discussion, or the number of channels that can be formed on IRC. As a user you run a "client" program like mIRC which connects to a "server" in an IRC network. All servers are interconnected and pass messages from user to user over the IRC network. One server can be connected to several other servers and up to hundreds of clients.