IRC, also known as internet relay chat, is an open protocol that can be used to create text based chat rooms online. It was created in the late 1980s by a man named Jarkko Oikarinen and was inspired by Bitnet Relay, a popular chat services provided by BITNET. IRC has been famously used during media blackouts such as the Soviet coup detat attempt back in 1991 as well as other major events such as the Gulf War.
To use IRC you must use an IRC client, such as mIRC, to connect to an IRC server. There are many IRC networks you can connect to which are made of many IRC servers connected together, allowing people from different servers to interact. Popular IRC networks include Undernet, EFNet and DalNet, and have servers located around the world.
IRC networks and servers are divided into channels, which are basically just separate chat rooms people can join to talk to each other. When you connect to an IRC server, you can use the command LIST to find all available channels. A GUI IRC client will likely have an option to list and find channels as well. Once you find an interesting channel you can use the JOIN command to connect your client to the channel. Once you have joined you will be able to send and receive messages to and from the channel. To stop receiving messages from the channel you must PART the channel, or typically you can just close the window in a GUI client.
Users and channels on IRC may have various modes implemented which are used to control various settings such as invisible mode for users, or invite only mode for a channel. Some mode commands are also used for security functions such as kicking and banning people from a channel. Typically people will use programmed bots to control channels and regulate the users who join.