In short, it's a system of controlling "intelligent" lighting fixtures and dimmers.
Technically, DMX is an abbreviation for DMX512-A, the ESTA (Entertainment Services Technology Association) Standard for controlling lighting equipment and related accessories. In January 2011 ESTA merged with PLASA, a similar organization in the UK. The new organization is called Plasa.
A wide variety of lighting control consoles, controllers and other devices that output DMX signals can be used to connect to an even greater variety of lighting fixtures and accessories that can be controlled by DMX. DMX controlled lighting systems are used in many professional settings, including concert lighting, stage lighting, studio lighting, theme park attractions, and much more.
In recent years, the DMX standard is being used more frequently in Architectural lighting projects, including illumination of building exteriors, accent lighting, general purpose building management and high-end residential lighting. This is due primarily to the high popularity of LED based lighting fixtures, which are frequently controlled via DMX signals.
Interactive Technologies produces DMX based devices for controlling and/or working with all kinds of DMX lighting fixtures. Our CueServer product family is a general purpose lighting controller that combines the power of a professional DMX lighting console with the tools necessary to build smart, self-contained lighting show playback systems. Our SceneStation product family is an easy-to-use and very capable wall-mounted button station that directly controls DMX lighting. The MicroTech DMX family of products has become an inductry standard for troubleshooting all kinds of DMX network problems.
If you're looking for more information about the DMX standard and related topics, there's a large amount of information available online. Here are a few links to get you started (they'll open in a new window/tab):