The IRC protocol (Internet Relay Chat) is one of the longest standing protocols on the internet. It is not run by any particular company, instead it is a standard, by which IRC clients can communicate with IRC servers. This protocol is implemented in Miranda through the IRC Plugin.
In IRC you log on to a server, of which there are many. Miranda comes with a list of servers pre-installed, but you can also add your own. The most popular are Undernet, QuakeNet, and Freenode.
Once logged on to a server, you can join channels (chat rooms), and join in conversations with people in that room. IRC also has a private messaging feature, for one-to-one chats.
The best thing about IRC is that on most servers you can easily create your own channels. Just create a name for it (usually begins with '#'), and 'Join' that channel. You are automatically made 'Op', or administrator of that channel.
If you would like to find out more about IRC, there is more information on irchelp.org.
Miranda has its own channel on Freenode, #miranda.