Command Control is a method of controlling model locomotives using various schemes, typically a high frequency signal (from audio to radio frequencies) is used to control a decoder located in the locomotive. Most Command Control systems are analog in nature, often employing a constant direct current voltage with a higher frequency, low voltage signal superimposed on it.
Digital Command Control uses a purely digital signal with no analog component on the rails. The locomotive mounted or track connected decoders are purely digital and respond to the DCC messages to generate the signals suitable for the locomotive motor, point motors, signals or other circuits. Doing so eliminates many of the problems and limitations inherent with analog systems. The NMRA has defined Digital Command Control through a number of published standards, there is room for future expansion and enhancement in the DCC Standard.
See Introduction to DCC for more information on DCC.