A client is anything connecting to a server that is not another server.
– J. Oikarinen, D. Reed; Internet Relay Chat Protocol;
When you log onto IRC with your client application,
it logs onto the server you set as your default
in the program settings. You can then join any public chat room running on that
server. Every message
you enter in a given chat room is then sent by your client application across the Internet to the IRC server, which immediately echoes it to every other user
connected to that chat room.
Because the server copies and echoes the messages across the world so fast, traveling across copper and fiber connections at 2/3 the speed of light, the illusion is maintained that everyone is directly connected to everyone else, even though
they are only really connected in common to the server.