Demos are programs that generate multimedia presentations in real-time on a high-end PC worth of about 1200 euros. Everything has to happen in real-time when demo is running and this separates demos from animations that are not limited with this real-time constraint.
In 64k intro competition these programs are limited in size to 64 kilobytes that gives enough space to generate desired graphics and music algorithmically but doesn't enable usage of drawn graphics or recorded music.
