Mobile phone access technologies

requency division multiple access (FDMA)
The first-generation analog mobile access method; FDMA uses separate frequencies for each call. For example if there are four stations, each will receive their own frequency. Thus there is no interference between them. This reduces interference, but limits the number of users.

Time division multiple access (TDMA)
TDMA allocates unique time slots to each user within each channel, thus allowing many users to access the radio frequency without anyone's interference. Each mobile conversation uses only a particular frequency slot of the time.
More information on TDMA

Code division multiple access (CDMA)
Commercially introduced in 1995 by Qualcomm, CDMA quickly became one of the world's fastest-growing wireless technologies. 3G CDMA networks provide more capacity for voice traffic, along with high-speed data capabilities, hence many cellular service providers are now building or upgrading to it.

CDMA is used in both 2G and 3G wireless technologies. Indian Cell operators like Reliance IndiaMobile, TATA Indicom, BSNL and MTNL's Garuda all use CDMA2000 technology and its variant CDMA1x.
More Information and White papers on CDMA
Faqs on CDMA

Global system for mobile communications (GSM)
Developed on TDMA protocol in Europe, GSM is now the worldwide standard technology for mobile communications. The advantage of GSM is that it provides a high level of security, globally accepted technology standards and superior sound quality.

For any GSM phone to work, it requires a Subscriber Identity Module (SIM), a card that contains your telephone account information. Thus, an Indian GSM phone can be used in USA and other countries too.
Faqs on GSM