There are two kinds of networks types: CDMA and GSM
CDMA: CDMA is predominantly limited to the US and parts of Asia. It is not supported throughout the European Union because of regulatory restrictions.
GSM: GSM is the a much more international standard and works across most geographies including the european union.
If you are looking to travel mostly inside US you should stick with CDMA because it offers a much better coverage than GSM, especially in the rural areas. However, if you plan to travel internationally, GSM might be a better option because of much broader support.
Each network type is further supported over different frequencies - also called as bands. The different bands include 800 MHz, 850 MHz, 900 MHz, 1800 MHz and 1900 MHz. Note that 800 MHz and 850 MHz are actually the same band and are used interchangeably. GSM Quad-band cell phones support the 850,900,1800 and 1900 bands and are best suited for international roaming.
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