This is a common question I receive. The first thing you need to understand is that these acronyms stand for two different types of cell phone technologies. The cell phone carrier (or company) that you have will dictate which type of phone that you can use, a GSM phone or a CDMA phone.
The first technology is GSM, or Groupe Special Mobile technology. GSM is the widest used worldwide cellular technology; more carriers have developed their networks on this standard. It was first developed in Europe in the late 1980's as a means to standardize cellular technology and allow easier roaming between networks in other European countries. Phones that are built on this standard have a sim card that is inserted into the phone. The sim card is the user's specific identification for that network. If a user wants to change phones, all they have to do is remove the sim card from their phone and place it into another phone that will accept that carrier's sim card. The user's identification on the network is transferred tot he new phone via the transfer of the sim card. The two biggest GSM carriers in the United States are AT&T and T-Mobile.
The second technology is CDMA, or Code Division Multiple Access technology. CDMA technology was developed by Qualcomm in the United States. Phones that are build on this standard are programmed with a unique ESN (Electronic Serial Number.) The carrier then assigns the user's telephone number to the ESN of that specific phone. The ESN is generally found on the back of the unit, visible when the battery is removed. In order to switch to a different phone on a CDMA network, the cell phone carrier has to assign the user's phone number to the new phone. The most widely known CDMA carriers in the United States are Alltel, Cricket, Metro PCS, Sprint, and Verizon.
