A method of controlling multiple trains independent of track voltage
Command Control allows the independant control of several trains without altering track voltages or using blocks. It is a generic term the describes many different systems offered over the past 60 years.
The first command control system was introduced in 1946 by Lionel. The next major leap was ASTRAC from General Electric, a transistorized system. These systems are best described as carrier command systems, or another term would be analog command control.
Others would follow, all using some form of high frequency carrier or tones carried on the track to control a locomotive. Analog in nature, these systems were costly, limited in features and often completely incompatible with any other system on the market. Even upgraded versions of a command control system were, in many cases, incompatible with the previous system, sharing only the name. The level of technical skill required to set up and maintain a command control system was often a liability that prevented widespread adoption. The first digital system used an analog computer, adding complexity to an already costly system.
Command control never gained a significant share in terms of modellers abandoning the traditional analog (Direct Current) control technology for it. Within that segment there was no clear leader either. The primary reasons were cost and compatibility.
DCC, or Digital Command Control, is the name applied to the NMRA's standard for digital command control. The term Digital was added to differentiate it from the other, mainly analog command control systems on the market at the time.
See DCC History for more info