IRC clients are computer programs that a user can install on their system.
These clients communicate with chat servers to transfer messages to other clients.
The chat rooms most relevant to English Wikipedia are listed hereafter; a more complete list of channels (for other language Wikipedias, other languages, and recent changes feeds) exists at m: IRC/Channels.
"Chat room" is a name for an Internet Relay Chat (IRC) channel.
Also, sometimes the reason it is busy is that the conversation has gone off-topic!