It's the older technology to deliver calls, texts, etc. to phones. In USA carriers like Verizon use it whereas AT&T uses more modern technology (I get all these abbreviations mixed up, if I tried to tell you which one/s I'd probably get it wrong).
http://en.wikipedia.org/wiki/CDMA
Here in Australia CDMA has been abandoned and towers have been shut down. This was announced in 2005-ish and new plans were made unavailable on CDMA, and now the towers have all (or mostly?) been shut down.
Somebody please correct me if I'm wrong somewhere!!
I look at CDMA as 2 things here in the forums (when people mention CDMA), a technology and a network. The technology has been around for a while, used a lot by the military. It's outstanding and by no means a dead technology. In fact, it's good enough to use as the air-interface for GSM. It scales very well (high call volume), no mapping of towers and covers a greater distance (less towers). As a network, CDMA is being replaced by LTE. My guess is the CDMA part will stay working for 10 to as many as 15 years. They have so much coverage, I wouldn't turn it off anytime in the near future.