Not locked to it or anything (plenty of efs backups) just wondering what differences there are between say, the US band and the Cellular 800 band.
The difference is in the range of frequencies that each carrier use. The usage of these frequencies is regulated by the Federal Communications Commission (FCC).
The usage of the different frequency bands varies around the globe although there is a large degree of standardisation. The GSM frequencies available depend upon the regulatory requirements for the particular country and the ITU (International Telecommunications Union) region in which the country is located.
For instance, in North America the USA uses both 850 and 1900 MHz bands, the actual band used is determined by the regulatory authorities and is dependent upon the area.
Since spectrum is a finite resource, the FCC distributes spectrum licenses geographically into a number of areas, and mobile operators must bid on each area individually. A license owner can use any technology within the licensed area and frequency range subject only to the band rules defining the limits.
Here are the charts that includes the 3G, 4G (LTE) network technologies, bands, and frequencies of all five major U.S. carriers:
NB: bandwidth — refer to the range of frequencies that can be contained within a signal.