The above sites will give you a good starting place for finding client software that works for your computer. A net, basically, is a collection of servers. If you are on Server A and your friend is on Server B, you can talk to each other if both servers are on the same net. As time grew on, the demo scene moved to the IBM PC world, and so did the MODs. As the PC power increased, so did the complexity of this tracked music and the power of the sampling engine.
Competitions abound for the music alone.
Thanks to the lovely and talented Chad Gould for kindly granting me permission to use his writing above. First, you need to be online. You also need an IRC client, a program which enables you to connect to the IRC network. Then you need to connect to the proper server. You have to have the sound yourself to hear it.