The demoscene is a computer art subculture that specializes in producing demos, which are audiovisual presentations that are run in real-time on a computer or other platform. Artists try to push systems' capabilities to the max trying to make their demos look technically and/or artistically as impressive as possible while usually trying to have catchy and memorable music in the background.
Originally started as cracktros, which were made by groups who managed to remove Copy Protection from software and added their own Loading Screen to brag about it. Simple cracktros were gradually replaced by more impressive ones. Finally, demos started appearing without being attached to games or other applications.
Technical capabilities of older demos and cracktros were limited by graphic and sound chips and available memory... but due to the ersatz programming that demoscene authors invented, often to nowhere near the same degree as actual games software on those systems.
Demos are often categorized into different groups based on their size and platform. The most common platforms of demos are PC (meaning MS-DOS until around 1999 and mostly Windows thereafter), Amiga, Commodore 64, ZX Spectrum and Atari home computers (both 16/32-bit (Atari ST-compatible) and 8-bit (400/800-compatible)). There are demos for almost any platform, though.
PC and Amiga demos are usually divided by size. There are demos which have very loose size restrictions and where the demo is only limited by the creator's imagination, then there are demos restricted by size. Size-restricted demos are usually called intros. The most common categories for size-restricted demos are 64 kilobytes and 4 kilobytes, although there are even smaller quality demos and competitions like 1-kilobyte, 256-byte, and 32 bytes or less (by which point they are typically DOS comfiles so as to dispense with executable headers). note Smaller demos usually rely heavily on procedural generation where assets such as 3D objects and textures are generated using simple algorithms.
When used as a genuine examination of how well a new system is expected to perform (for attracting interest at trade-shows and the like), such processes are typically referred to as technology demos. Technology demos are usually made by the hardware companies themselves, however, not the demoscene.
As a side-note, size-limited demoscene productions typically use obscure executable compression which, outside the demoscene, is only ever used to cloak malware. It is therefore not unusual for antivirus or antimalware software to report clean demoscene files as infected. Likewise, it can be difficult to tell if something actually is infected.