CDMA stands for Code-Division Multiple Access, which is a digital cellular technology that uses spread-spectrum techniques.
Unlike competing systems such as GSM, that uses TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence.
CDMA is a form of spread spectrum, which simply means that the data is sent in small pieces over a number of the discrete frequencies available for the use at any time in the specified range.
All of the users transmit in the same wide-band chunk of spectrum. Each user's signal is spread over the entire bandwidth by a unique spreading code. At the receiver end, that same unique code is used to recover the signal.
CDMA is a military technology first used during the World War II by the English allies to foil German attempts at jamming transmission. The allies decided to transmit over several frequencies, instead of one, making it difficult for the Germans to pick up the complete signal.