A demo is a program that displays a sound, music, and light show. Demos are very fun to watch, because they seemingly do things that aren't possible on the machine they were programmed on. Essentially, demos "show off". Demos are an art form. They blend mathematics, programming skill, and creativity into something incredible to watch and listen to.
Programming Technique + Art = Demo

This category is not for portfolios of multimedia art but for the specific artform of demos, also known as eurodemos, a program that displays a sound, music, and light show.

Often persons join together in demo groups where they have responsibility for various parts of the demo. Some may do the music, others code while other may do graphics. This category covers the various groups active in the demo scene.
Please place sites in the proper category according to platform.
About the music used in demos. Often the music is written in the MOD format that originated on the Amiga. The program used to write MODs is called a tracker after SoundTracker by Karsten Obarski that was the original program (actually MegaTraxx existed earlier but never became popular).
If you don''t know what a MOD and a tracker is you are probably in the wrong category.
A demo party is a event in which many members of the demo scene gather to exchange knowledge, compete in various contests to demonstrate their skills and generally have a good time. Most demo parties are held in Europe.
Note: LAN parties, in which people just play games and spread warez and MP3 over a network, don''t belong here.

PC

Demos running on PCs running either MS-DOS or Windows in any flavour.
This category covers the art of demo programming. Demo programming is similar to programming games as they both require fast graphics but the interactive element is missing or at least very limited.