GSM and CDMA are two different but similar wireless technologies. GSM enjoys about an 82% market share globally. In the US CDMA is the more dominant standard.
So you may ask which is the best and why ? Here in the UK we use GSM. Because GSM is used in more than 74% of the markets across the globe, users of tri-band or quad-band handsets can travel to Europe, India, and most of Asia and still use their mobile phones. CDMA however offers no multiband capability, however, and therefore you can’t readily use it in multiple countries. However, certain phones like the Blackberry Tour and the HTC Touch Pro 2 now have Quad-band GSM built in so they can be used overseas with special calling plans from mobile operators.
Another big difference between GSM and CDMA is in the data transfer methods. GSM uses high-speed wireless data technology known as GPRS (General Packet Radio Service), this offers a slower data bandwidth for wireless data connection than CDMA’s high-speed technology (1xRTT, short for single carrier radio transmission technology), which has the capability of providing ISDN (Integrated Services Digital Network)-like speeds of as much as 144Kbps (kilobits per second). However, 1xRTT requires a dedicated connection to the network for use, whereas GPRS sends in packets, which means that data calls made on a GSM handset don’t block out voice calls like they do on CDMA phones.
GSM networks operate on number of different carrier frequency ranges most 2G GSM networks operate in the 900 MHz or 1800 MHz bands. Where these bands were already allocated, the 850 MHz and 1900 MHz bands were used instead (for example in Canada and the United States). Most 3G networks in Europe operate in the 2100 MHz frequency band.
Regardless of the frequency selected by a mobile phone operator, it is divided into timeslots for individual phones to use. This will allow for eight full-rate or sixteen half-rate speech channels per radio frequency. These eight radio timeslots (or eight burst periods) are grouped into a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate for all 8 channels is 270.833 kbit/s, and the frame duration is 4.615 ms. The transmission power in the handset itself is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900 phones.
There is a good document here produced by OFCOM that explains how a mobile phone network works.