DMX - Digital Multiplex
DMX is based on an old American protocol know as rs485.
Traditionally DMX comm cables are terminated at each end with a 5 pin xlr plug (male one end and female at the other). Only 3 of these pins are actually used and as a result many lighting fixtures now have only 3 pin xlr connections although some do have both types.
The wiring of either type of plug is normally p1=screen p2=negative p3=positive 1). As I said this is the NORMAL way but you should know that some fixtures will be p2=positive p3=negative. P1 is ALWAYS screen. There are some fixtures on the market which are auto detect, meaning it will not affect anything if the positive signal appears on p2 or p3. Always read your fixture manual to discover what your fixture is looking for. Incorrect communication will result in very strange or even no control.
Basically a single dmx universe comprises of 512 possible communication channels, with each channel having a variable value of between 0 and 255. A lighting fixture such as a moving head may have numerous channels, pan, tilt, colour wheel, gobo etc. Each of these attributes has its own channel and depending on the level given on that channel will produce different results.