Internet Relay Chat (IRC) is one of the oldest chat platforms on the Internet, and mIRC is one of the best known IRC chat clients.
The technology is mostly about holding real-time group chats in chat rooms, called channels (although you can have private chats with individual IRC users, too). It's far more complex than just chatting in the average web chatroom, but then you can also do more, and mIRC generally does a good job of walking you through the initial IRC basics.
Once you're chatting you'll discover that the program has all the extras that you'd expect in any modern messaging client: buddy lists, file transfers, message logging, SSL encryption, IPv6 support, and more.
If you want to go further, then the program can be automated through its own scripting language. This is no toy - it supports DLLs, can call COM objects, create graphical windows, process binary files, create sockets for network communications, and more - and can do anything from manage your IRC channels to playing complex multi-user online games.
And while the program isn't free, the licensing is just about as generous as you'll get. A single-user licence of $20 will allow you to install mIRC on up to 3 computers, and it's a lifetime deal - you'll never have to pay again for any future version.