Using your GSM Cell Phone around the world

What is GSM?
GSM is the abbreviation for Global System for Mobile Communications. GSM operates on the multiple spectrum frequency bands used around the world - 1900MHz, which is used in North America and 900MHz and 1800 Mhz for other locations.

In the USA, not all mobile service providers operate on GSM. If you have T-Mobile and AT&T (there are others) you are in luck, you are on a GSM network, but the GSM frequency used in the US is not the same in other countries - you will need a phone that can operate on the different frequencies used around the world. The best phone for travel is a quad-band phone which will allow you to use your phone anywhere in the world.
Tip: Make sure that your cell phone is compatible with the GSM band of the country you will use it in.
 
GSM frequencies and countries
900 Mhz and 1800 Mhz - Europe, Africa, Asia, Australia and much of South America
850 Mhz and 1900 Mhz - North America and South America

In the US, different service providers operate on different frequencies within the country. For example T-Mobile operates on 1900 mhz in the USA, while Cingular operates on 1900 mhz or 850 mhz.

Quad, Tri, and Dual band cell phones
Quad-band - A quad-band phone allows you to roam almost anywhere globally. It covers the 850 Mhz, 900 MHz, 1800 MHz and 1900 MHz frequency ranges. Many countries in South America only have 850 MHz GSM service while both 1900 MHZ and 850 MHz GSM services are supported in the USA. Quad band phones will work all over the world with all GSM service providers.
Tri-band - A tri-band phone operates on three frequency bands, 900 MHz, 1800 MHz and 1900 MHz, allowing you to use it in Europe, Africa, Asia, nearly all of North America, Australia and New Zealand.
Dual-band - A dual-band phone operates at both the 900 MHz and 1800 MHz spectrum. Dual-band works in Europe, Africa, Asia, Australia, New Zealand and much of South America but will not work in North America.

But wait, there's more - SIM cards
So, you have a quad-band phone. If you want to keep the same phone number and service that you use in the US, you will need an international plan to keep your calling rates in check. This is still going to be expensive. If you want to save money, buy a SIM card (SIM=Subscriber Identity Module) that will work in the country or countries you are visiting.  It is best to get the SIM card before you go. Read all about it at goeurope.about.com 
Your cell phone will have to be unlocked to use a SIM card from another service provider.

Unlocked cell phones
Normally all the phones sold by the service providers are locked to be used with their service only. Unlocked phones are not locked to any specific provider and will work with any GSM service provider that is supported by the phone's frequency band. You can usually get an unlock code from your service provider which you type into the phone using the dialing pad.