Talk is done in real time, with users typing their messages back and forth on their keyboards.
There is no limit to the number of people that can participate in a given discussion, although it can get rather confusing if too many people are talking at once on the same channel. All servers are interconnected and pass messages from user to user over the IRC network. There are many different IRC networks, some serving as many as 15,000 or more users all at once. For Mac users, I recommend Ircle . It's shareware and can be downloaded here .