The Demoscene is a computer-art subculture that specializes in producing demos, which are non-interactive audio-visual presentations that run in real-time on a computer. The main goal of a demo is to show off programming, artistic, and musical skills (wikipedia).
Starting on the Commodore 64 during the 8-bit era, The Demoscene was created by crackers putting their own intros on cracked games. This practice has evolved beyond cracking games to its own art form. Now demos, as they are called, are all about a audio video presentation in real time. The key is the real time part; these aren’t pre-rendered videos. They are computer programs that are rendered and shown in real time through C and C++ programming.