While Internet Relay Chat is the system for chatting, mIRC is one of the software applications used for IRC. According to www.mirc.net (where you can download mIRC), "no one is quite sure what the 'm' in 'mIRC' stands for." To join an IRC discussion, you need an IRC client such as mIRC, a program that runs on your computer and sends and receives messages to and from an IRC server.