Definition of "frequency band"
Frequency bands are groupings of radio frequencies that are used by mobile networks to communicate with mobile phones. The frequency bands that a phone supports determine to a large degree where and on which networks it can be used. For example, T-Mobile USA uses the 1900MHz band for its GSM network, while AT&T uses both 1900MHz and the newer 850MHz frequency bands. Europe and most of the rest of the world use the 900MHz and 1800MHz frequency bands for GSM. So for a device to work properly on T-Mobile USA as well as work in Europe, it would need to support the 1900MHz band and one or both of the 900MHz and 1800MHz bands used in Europe. A tri-band device supports 3 different bands, while a quad-band device supports 4.