CDMA, or Code-Division Multiple Access, is actually just a channel access method standard. When most people (including us) say CDMA, they usually mean CDMA2000, which is a family of 3G communication standards that run on top of the CDMA mechanism of utilizing airwaves. Confused? Unless you're an RF engineer, the distinction probably doesn't matter to you.
In the US, Sprint and Verizon use CDMA. T-Mobile and AT&T use GSM.
Worldwide, GSM (which stands for Global System for Mobile Communication) is far more popular. But CDMA is still used in 40 countries, primarily in Asia. China Telecom is the next largest CDMA carrier, with over 90 million subscribers.