An IRC client is a software application used to chat with other people in Internet using your keyboard.
The users of IRC clients connect to an IRC server where they can join thousands of public channels dedicated to all kind of topics. In those channels you chat with a group of people about the topic of that channel but you can also create private channels to chat with your friends.
Each user is identified by a unique nickname and certain privileges are assigned to that user in each channel that allows him/her to talk, remain visible, be a channel operator and receive messages from the server or wallops.
Channel operators can kick other users out of the channel, change the privileges of other users and change the title of the channel, among many other things. If a user creates a channel, he/she becomes the channel operator.
The IRC servers are grouped in networks that share channels, users and regulations. Some of the most famous networks are freenode, EFnet, IRCnet, Undernet, DALnet, QuakeNet, Rizon and IRC-Hispano.