GSM phones are more common outside of the United States, while CDMA phone remain popular in the United States because of Verizon and Sprint. GSM phones use SIM cards, while CDMA phones use built-in identifying numbers.
The GSM standard was created in 1987 by a consortium of companies, and is nearly ubiquitous in Europe and popular elsewhere. Qualcomm was largely responsible for creating CDMA, so it can be more expensive to license. Verizon and Sprint use CDMA primarily because it offered advanced features in the mid-1990s when both companies were retooling their networks.
Unlocked GSM phones are easier to move between different providers. While carriers can sell phones locked to their networks, most provide a means of unlocking phones after users fulfill certain contract requirements. Always check a phone's GSM capabilities before switching, as providers use different frequencies. CDMA-based providers use a whitelist to allow only authorized phones to access their networks. Although a carrier can add a non-whitelisted phone to its network, this practice is uncommon.
Both GSM and CDMA phones are 3G technology, and the four major carriers in the United States offer LTE connectivity. However, 3G towers serve as a fallback when LTE service isn't available. Sprint and Verizon also use SIM cards as a means of authenticating devices for LTE connectivity.