Motherboards are circuit boards found inside electronic devices. They hold all the necessary circuitry for basic functionality of the device. PC motherboards generally come in a form factor, or size and shape, called ATX. Mini-ITX is a motherboard form factor designed to produce computers smaller and quieter than typical desktops and towers.
The Mini-ITX standard was created in 2001 by VIA Technologies. The traditional ATX board is 300mm x 240mm; a mini-ITX board is barely one-third the size, 170mm x 170mm. The size reduction is achieved by removing a number of expansion slots and soldering components to the motherboard instead of attaching them with bulky sockets.
Mini-ITX motherboards also use processors, or CPUs, that are one or two generations behind the state of the art. The CPUs are not made by Intel, but are clones of Intel's Pentium architecture and retain full compatibility. In addition to being smaller than newer processors, they are run at a much slower speed. In semiconductors, the amount of heat generated by a device is proportional to the speed it runs at; by accepting a lower level of performance, Mini-ITX motherboards can eliminate most of the bulky cooling devices found in ATX systems. In particular, they can be built without fans.
PCs based on mini-ITX take advantage of the small size to create computers that look completely unlike the "beige box" so prevalent in offices and homes. The small size and simple cooling requirements allow a computer to be built out of almost anything; mini-ITX boards have been mounted inside toasters, antique wooden radios, and Art Deco jewelery boxes. One intrepid individual even built a computer inside a George Foreman grill. Because the lack of whirring fans allows silent operation, mini-ITX boards are often found in computer-based home theater systems.
VIA Technologies is working on an even smaller motherboard standard called nano-ITX. It will be a mere 120mm x 120mm, or roughly 5 inches on a side. Computers built using nano-ITX will be roughly the size of a paperback novel.