To add what dchawk81 said,
A true world phone would have a GSM chip set that covers 900mhz and 1.8ghz.(1800mhz) Thats for over seas, in the US we use 800mhz (sometimes labeled 850mhz) and 1900mhz. If you see a device labled as Quad Band GSM then it covers the 2 bands in the US and the 2 bands found in the rest of the world. Most ATT / TMOBILE devices have these capabilities due to them already having GSM chipset in them. Sprint and Verizon are CDMA 800/1900mhz.