CDMA stands for Code Division Multi Access. What happens with this service is that many conversations share one cell but based on the code assigned to the discussion will determine at which section your conversation packet will enter the stream to the cell at the tower, then passed to the land lines and to the receivers phone.
GSM stands for Global Satellite Messaging and conversations are passed from the closest cell tower, to a satellite, and then to your receiving party.
CDMA has been adopted by IEEE (I believe) to be the standard for future broadband - can't remember the specifics. However, Verizon is the only main stream cellular provider I know of that uses CDMA. The others (AT&T, Cingular, TMobile) use GSM as their main standard.