Demos are programs that generate multimedia presentations in real-time while demo is running. This separates demos from animations that are not limited with this real-time constraint.

In 4k intro competition these programs are limited in size to maximum 4096 bytes on 2002 PCs, Amigas, video game consoles and Java VM. 4096 bytes is comparable to the amount of text on one A4 sized page and therefore takes real skill to fit great effects and music to that size.