IRC is a chat service, like AIM or Yahoo, or ICQ; it's one of the oldest chat services on the internet.
An IRC CLient allows you to join an IRC Server; the pJIRC addon at UBBDev is a way to integrate an IRC Server into your forum.
You can set the addon to allow you to join any server (there are millions, see
SearchIRC for an example)
The Trivia channel is simply a channel on the chat server, which is geared towards trivia games; I plan to introduce other options in the future on the server as well.