Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies.
CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies. To permit this without undue interference between the users, CDMA employs spread spectrum technology and a special coding scheme (where each transmitter is assigned a code).
CDMA is used as the access method in many mobile phone standards. IS-95, also called "cdmaOne", and its 3G evolution CDMA2000, are often simply referred to as "CDMA", but UMTS, the 3G standard used by GSM carriers, also uses "wideband CDMA", or W-CDMA, as well as TD-CDMA and TD-SCDMA, as its radio technologies.
CDMA phones may or may not have SIM cards, but the user information is stored with the service provider, which must give its permission for you to switch phones. CDMA phones must be programmed with every carrier you use.
Whenever you switch carriers, the phone must be reprogrammed for that carrier - even if it is an unlocked phone.