IRC is the most full featured chat client

IRC is yet another way to chat across the internet. It actually consists of many different servers, each supporting many different people chatting.

Although IRC began on Unix, you can get a client for many different operating systems. 

IRC is based upon the concept of channels, similar in concept to the chatroom of other packages. You simply find a channel which is about a subject that you want to chat about, and connect to it. At any one time there may be thousands of channels going at once, each about a different subject.

Chat channels typically begin with a pound sign.  An example would be #loveroom. 

IRC is very powerful, but it is somewhat difficult to learn. Users with Unix experience will be right at home, while others may be a bit put off with the typical command line interface.

Personally, I have not used IRC very much. I've preferred Instant messenger and ICQ, which I find easier to use and with a larger user base.