In the early 1990's, the American National Model Railroader's Association (NMRA) recognised the need for a set of published standards and recommended practices which define Digital Command Control (DCC).
These standards define the capabilities and technical specifications and implementation of a digital system which operates between a command station and a loco or accessory.
Today, these standards are known as 'NMRA DCC' or just 'DCC' for short and they are internationally agreed.
It should be noted that there are other digital standards. In Europe, Selectrix is an NEM standard, but the Märklin-Motorola system is proprietary and used only in Märklin products. In the US the Rail-Lynx system provides power with a fixed voltage to the rails while commands are sent digitally using infrared light.
Other systems include the Digital Command System and Trainmaster Command Control.
NMRS DCC is the most common system. It was originally developed by Lenz of Germany for Märklin, and they allowed the system to become an open standard. DCC has been adopted by both the NMRA and the NEM as a standard. Several major manufacturers (including Märklin and latterly Hornby), have entered the DCC market alongside makers which specialize in it (including Lenz, Digitrax, NCE Power Pro and CVP Products' EasyDCC, and Train Control Systems).
DCC provides a standard digital network architecture which forms that basic architecture of a system to which many other products can be connected and provided they are compliant with the NMRA standards, products from different manufacturers will be compatible and operate together.