These are the technologies I know of:

GSM is most common in Europe. GSM phones have SIM cards. There are three differenet frequencies which GSM works with. Two of them are common in Europe, the third is used in the USA. I'm not sure about the rest of the world, but I know that a phone that supports all three frequencies (TriBand phone or "World phone") can work in most densely-populated areas in the world. My dad has a TriBand GSM phone - Nokia 6310i.

CDMA is common in Japan.
TDMA is most common in the USA, methinks.
iDEN is a Motorola technology which allows using two phones for "Walkie-Talkie".


I have a TDMA phone - Nokia 7160..... My next phone will be a GSM Nokia though. You see, my mobile service provider, Cellcom, runs both a TDMA  *and*  a GSM network (with GPRS and HSCSD support ).