The CDMA and GSM technologies define the most basic functions of your phone. GSM (Global System for Mobile Communications) and CDMA (Code Division Multiple Access in the context of cellphones and mobile networks), they are both shorthand for different mobile phone technologies.
These technologies convert your data into radio waves that your cellphone sends and receives. GSM divides the frequency bands into multiple channels so that more than one user can place a call through a tower at the same time. Meanwhile, CDMA networks layer digitized calls over one another, and unpack them on the back end with sequence codes.
In the U.S. there are more CDMA users than GSM users, the two largest CDMA carriers have over 43% of the market share. The two largest GSM carriers barely break 37%; worldwide, CDMA accounts for around 13% of phones, with GSM and its successor, UMTS making up of the remainder.
CDMA carriers in the US:
Verizon, Sprint and whoever uses their networks (Virgin, Boost, Alltel)
GSM carriers in the US:
AT&T and T-Mobile
Both GSM and CDMA standards gives out information so that the phones are identified by carriers. GSM use a removable chip called a SIM card. If you are using an openline phone you can just remove the SIM card and transfer it to another openline phones with ease. The CDMA standard have RUIM (removable user ID module) these phones are locked to one network, and can only be switched to another with the approval of the both the old and new carriers.
In the US, phones are sold with contracts and discarded with after two years. However, in other countries phones are sold unlocked, and works with any and all local networks. If you are to choose between CDMA vs. GSM for travel GSM always wins.