IRC stands for Internet Relay Chat. Detailed information can be found at Wikipedia:Internet Relay Chat.
IRC allows one-line text messages between users through software clients to connect to servers offering permanent or temporary channels (forums) as well as one-on-one messaging between users and bots. There is a rich infrastructure of server, client, bot, and other software. Levels of administrative capabilities allow management of channels, users, servers, and networks.
There are multiple established networks with permanent and ad hoc channels, such as EFnet, Undernet, Freenod, DALnet.
Server software is readily available for Linux and other operating systems.
Client software exists for desktop Windows, MacOS, Linux, and BSD operating systems, in GUI, console, commandline, and programmatic versions as well as iOS and Android mobile devices.
While still used extensively, especially among technical users, there's been a 60% decline in general usage from 2003-2012. Products such as Slack are somewhat glorified IRC systems. Some of the decline is attributed to protocol stagnation, with a 2016 effort underway for the IRCv3 protocol.