Code division multiple access
Code Division Multiple Access (CDMA) is a wireless communication system used by a variety of technologies. It is a multiple access system, enabling multiple senders to transmit on a single channel at the same time using "spread-spectrum" multiplexing. This is most commonly used by cellular phones, specifically those serviced by the Verizon (including U.S. Cellular) and Sprint (including Virgin Mobile and Boost Mobile) networks.
The most notable difference between GSM and CDMA technology to the end-user is the fact that they are not compatible with each other. Although some phones offer cross-compatibility, most do not, meaning that switching from one type of network to another requires a replacement phone. Other than this, there may be little if any apparent difference between CDMA and GSM systems. While this is largely true since the communication method is processed in the background, there are a few differences which the user can see.
Unlike GSM networks, CDMA does not requires a SIM card. When connecting to a 3G network, a CDMA phone identifies it with a hard-coded identifier (ESN or MEID), which is compared by the carrier to a whitelist. Some CDMA phones will have SIM cards, however, since that is the standard for the newer LTE technology. 
3G simultaneous access
Another notable characteristic of many CDMA cell phone systems is that they cannot process voice and data communication at the same time. A solution to this issue is available, but not always used. Additionally, the speed itself of CDMA 3G communication tends to be slower than that of GSM 3G service.
Although CDMA was a fantastic new innovation when released, times have changed. Access points for CDMA phones are good, but compared to the newer GSM technology, cannot service as many concurrent clients as GSM. This means that access points can become more easily overloaded, and individuals in high-use areas could be dropped or experience other quality issues if there is not a sufficient amount of access points to deal with the high quantity of users.