GSM, you ignorant goof tard. Not really, but I don't care for it much. GSM is an acronym for Global System for Mobile Communication. GSM works off of the older TDMA systems in using timestamps to stack calls on the tower. GSM can stack seven to ten calls per megahertz versus upwards of 30 for CDMA.
GSM is used almost exclusively in Europe making it more of the, "world standard" than CDMA. Many carriers sell, "World Phones" that are tri-band and capable of using the networks here in the States and in Europe. Data speeds on GSM are significantly slower than CDMA, and while new technologies are being developed to bring up the transfer rates, it will never really catch up with CDMA, much less overtake it.
For coverage we look to the number one wireless phone carrier in the United States, Cingular. Cingular recently acquired AT&T, and as their new marketing campaign is screaming, they now have the largest network. What they aren't telling you is that most of AT&T's network is TDMA and does not support GSM. AT&T was starting upgrades but never finished. Their coverage is very, very good. For coverage, their only competition is Verizon. However, CDMA penetrates (lol) buildings much better than GSM.
T-mobile is also a player. Instead of having a zomg huge network, they offer fantastic values in phones and service plans. T-mobile is great if you're in a metropolitan area and never leave.
Just incase anyone is wondering, all of the carriers have coverage maps on their respective websites. Additionally, tons more information about both CDMA and GSM are available from our friends and Google. Feel free to post questions in the shoutbox or in comments and I'll do what I can to answer them. And now I'm going to bed. Mmm. Bed.