actually mirc is a windows client which allows you to irc. irc is internet relay chat. incidental to the chatting you can also send and receive files and set up file servers for other people to be able to download files from you.
you get many irc networks around the world. other irc clients include pirch, trillian allows you to irc as well, and klient.
websites :
www.mirc.com
:
www.klient.com
:
www.pirchat.com
any questions about irc, feel free to pm me