CDMA and GSM are competing network technologies. Most cellular carriers use one or the other. GSM phones use interchangeable subscriber identity module (SIM) cards that identify each phone with a specific user or phone number. If you want to change phones, you can simply install your SIM card in a new phone. Unlocked cell phones will accept SIM cards from other carriers. CDMA cell phones don't use SIM cards, so an unlocked CDMA cell phone has to be programmed to work with each carrier you use.