CDMA is a digital wireless technology. It is a general type of technology, implemented in many specific technologies. But the term "CDMA" is also commonly used to refer to one specific implementation: IS-95 - a mobile-phone technology that competes with technologies such as GSM.
CDMA is a "spread spectrum" technology, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal.
Unlike many competing technologies, CDMA has no hard limit for the number of users who may share one base station (tower). Instead, with CDMA, additional users can connect until the base station determines that call quality would suffer behind a set limit.
CDMA systems have been in commercial operation since 1995. CDMA networks operate in the 800 and 1900 MHz frequency bands with primary markets in the Americas and Asia. IS-95 CDMA systems are sometimes referred to as cdmaOne. The next evolutionary step for CDMA to 3G services is cdma2000.