I think you are making a wrong comparison here, should the question be 'what is the difference between CDMA and GSM'.
General Packet Radio Service (GPRS) is a mobile data service available to users of GSM mobile phones. It is often described as "2.5G", that is, a technology between the second (2G) and third (3G) generations of mobile telephony. It provides moderate speed data transfer, by using unused TDMA channels in the GSM network. Originally there was some thought to extend GPRS to cover other standards, but instead those networks are being converted to use the GSM standard, so that is the only kind of network where GPRS is in use. GPRS is integrated into GSM standards releases starting with Release 97 and onwards.
Whereas the difference between CDMA and GSM can be said as;
One of the basic things that differentiates GSM (based on TDMA) and CDMA is the way the carve up bandwidth.
Each bit of radio spectrum used by a handset has to be shared with other people in the area. It is more or less the same as multiplexing for normal data land lines.
The major difference between GSM/TDMA and CDMA is in the way they divide up those signals between multiple users.
GSM/TDMA uses a Time Division method. TDMA, in fact, stands for Time Division Multiple Access. Simply put, this means that each device on the local network is allocated a time slice where it "owns" the bandwidth, and it can send/receive its data.
So lets just pick a number and say there are 30 available time slices in a given cycle. Each phone would then get 1/30th of every cycle that it could send and receive data (aka, voice).
CDMA uses a different method, called Code Division Multiple Access. The specifics of how it breaks the cycle up are beyond me, but how it works out is that the phones only get a slice of the bandwidth cycle when they actually need one. So if you are not talking, and the other person is not talking, nothing is transmitted.
With GSM/TDMA, each phone is transmitting and receiving during its slices of the bandwidth cycle, whether it needs it or not.
Since most coversations are comprised largely of silence, the end result is that CDMA phones have to transmit less data. They don't have to send silence, like GSM/TDMA phones do.
This means a few things. More CDMA calls can be fit into a given amount of frequency spectrum (ie. it is more efficient for the network), less radiation is being created from the phone towards the user (you only get radiation when you are talking, basically), and battery power is conserved since the handset only transmits when it actually has something to send.
There are other differences, too, that I can't get into. For one, it is harder to implement a CDMA network. The tower placement is more difficult. Dealing with hills is more difficult than with GSM/TDMA. Things like that.
But in general, CDMA is vastly superior technology. Not surprising since Qualcomm's version of it (that which is used in CDMA and WCDMA phones) is newer technology, even if the basics were in use by the US military as far back as the 40s.
The important thing for me, though, is the SIM card. Had CDMA implemented the use of a SIM card (something it very easily could have done), then the North American market would be very different today, and there would be more CDMA networks in other countries. The SIM card allows people to easily switch phones, and that helps the market.
There are also other differences: some CDMA cell phones usually include analog capability (especially those from Verizon), so that the phone can be used to make calls when the user is not in a digital cellular service area. But GSM phones usually don't offer that capacity. In Europe, this is not a problem, as digital service is available mostly everywhere, but in North-America, this is an issue for those who need to travel occasionally to the country, where digital service is rarely available. However, GSM phones carry other advantages over CDMA. For instance, triband or biband GSM phones (those that can operate on more than one frequency band) let you use your phone in Europe and the rest of the world while you can't do this with a CDMA phone.
Data Transfer Speed: With the advent of cellular phones doing double and triple duty as streaming video devices, podcast receivers and email devices, speed is important to those who use the phone for more than making calls. CDMA has been traditionally faster than GSM, though both technologies continue to rapidly leapfrog along this path. Both boast "3G" standards, or 3rd generation technologies.