When you join a channel, you can read the messages that are being typed in by other users and respond whenever you want. Your messages are immediately transmitted to all the other people on the channel so that they can reply in turn.
This immediacy is the chat system's great strength. Russian students used IRC to provide the entire world with instant updates during the attempted coup in Russia in 1993, and thousands of people tuned in to a channel coming out of the Middle East during the Gulf war.
For the most part, though, chat tends to be socially oriented, and there are lots of channels where people can discuss their favourite bands or TV shows, or try to chat up members of the opposite (or same) sex.
Be aware that chatting will not allow you to be as selective with incoming messages as a newsgroup. CompuServe and America Online run their own system of chat "rooms", which may be less clogged, but these are limited to CompuServe or AOL members.