GSM phones

GSM (Global System for Mobile communications) operate in 4 frequencies.
850, 900, 1800, 1900 MHz

If a phone is quad band phone, it supports all 4 frequencies. so it works any where in the world.

If it is not quad band phone we need to check what frequencies the phone supports and check if the operators in the area where you are planning to use the phone operates in that frequencies.

Suppose if you have a tri band phone which operates in 850/1800/1900, the phone doesnt works with operators that use 900 frequency.

In US ATT operates with both 850 and 1900 frequencies and T-Mobile operates at 1900.
In India Airtel, BSNL, IDEA operates at 900 frequency and Vodafone, Reliance, tata indicom operates at 1800 frequency.

If you take a tri band phone with 850/1800/1900 phone which worked in US with ATT or T-mobile, In india this phone works only with Vodafone, Reliance, tata indicom. it doesnt works with Airtel, BSNL, IDEA.

check the below link for more information on frequencies operated by various service provides in different countries.

http://www.gsmworld.com/roaming/gsminfo/index.shtml