CDMA and GSM are competing cellphone radio systems. While GSM networks use SIM cards to identify users, CDMA networks use whitelists. Even if a CDMA phone uses a SIM card, it's not for identification purposes.
GSM carriers store customer information on a removable SIM card, making it easier to switch carriers and phones. Putting the SIM card in another phone activates it instantly. Likewise, using a SIM card issued by another network makes the phone send and receive data through that network. Because CDMA networks use white lists as a form of identification, users can only switch phones and carriers with the carrier's permission, If a phone made for CDMA networks also has a SIM card slot, it is not to verify the user but to connect to 4G LTE networks.
CDMA networks cannot normally transmit data and calls at the same time. Although there is a technology called Simultaneous Voice and Data Optimization, or SV-DO, that enables CDMA networks to do this, carriers in the U.S. have not adopted the technology. In comparison, simultaneous call and data transfer is a required part of the 3G GSM technology.
GSM technology has improved faster than CDMA and is capable of higher data transfer rates. GSM networks that use HSPA can work as fast as 42 megabits per second. In comparison, CDMA networks can only transfer 3.6 megabits per second.