IRC is "very different" in the sense that it's more open and more freewheeling than, say, Slack. Slack is basically IRC with a slicker coat of paint and more monetization.
But I think I'm getting the idea now. It's a similar idea to a typical instant messenger, except that since the protocol is standard, you have federation that actually works, much like someone who uses gmail can email someone else who uses yahoo.
Plus you have the concept of a channel, which seems to have no direct analog in a typical instant messenger, but seems to be essentially a persistent chatroom that doesn't vanish even if nobody is in the channel at the moment.
Is that about right?
Also these persistent channels are created just by joining the channel with that name; they disappear when the last person leaves. Notably, the first person to join a chatroom gets "operator" status, and thus the rights to change channel settings and kick/ban people etc. So there evolved a culture of bots, who would sit in channels with op status and 'occupy' it in case someone came along; these bots had op status themselves, and were programmed to auto-ban certain users or even domains, and auto-op their owners and their owners' friends. slap me summa dat +o.
And of course, there were the bot wars and the attempts at deliberate netsplits to try and steal op status, which were fun games for the script kiddies, but would really take the piss out of your evening chat when they would come by.
shifto slaps rabidrat around a bit with a large trout
* <nick> <action text>
More generally, on most clients you could use "/me <text" command to do the same thing. And on the protocol level, it was done by a mini-protocol on top of IRC, called DCC (direct client-connection), that would look like:
PRIVMSG #channel :^AACTION text^A
(where "PRIVMSG #channel :text" was a way to send a normal message, and ^A is Ctrl-A).
DCC was also used as signalling for file transfer. Clients wishing to send files would open a high-end port on their machine and send a DCC message with the file name, IP and port to connect to (similar to active FTP).
(yes, I spent an inordinate amount on time on IRC...)