The United States cellular market is an open market subject to little government regulation, as is the case with many industries in the U.S. This free market atmosphere provides the consumer with the benefits of lowered costs through competition while sacrificing the uniformity and standards of service that often exist in other areas of the world. Carriers in the U.S. are free to develop new, competing standards which aren't always compatible with infrastructure in other countries.
If you plan on traveling outside of the U.S., you should sign up with a U.S. carrier utilizing GSM technology and purchase what's known as a Quad-Band phone (commonly referred to as a "world phone"). While many cell phone users in the U.S. subscribe to Sprint and Verizon's CMDA (or TMDA) networks or Nextel's iDEN network, most of the rest of the world uses GSM. AS such, if you plan to use your cell phone internationally, you'll want to sign up with a provider that uses GSM and offers international roaming services (either on their network or through partnerships with providers in other countries.)
AT&T, T-Mobile, and SunCom are the major providers using GSM within the U.S. While their networks in the U.S. operate mainly on the 1900 and 850 MHz bands, a Quad-Band phone can function on any of the four bands used throughout the world: 850, 900, 1800, and 1900 MHz. Not all GSM phones are Quand-Band, so make sure you're getting what you'll need when purchasing a new phone. Conversely, many import phones sold in the U.S. are Tri-Band 900/1800/1900 only; these phones will work on AT&T, T-Mobile, and SunCom's networks in many areas, but some less populated places are covered only by the 850 band. For more information, see ?Chapter 10 ? Understanding Cellular Technology and Frequencies?.