AT&T and T-Mobile are the two main providers (in the USA) who provide GSM service. Below are the main cellular bands they use.

AT&T:
GSM: 850 and 1900
3G: 850 and 1900

T-Mobile:
GSM 1900
3G: 1700 (AWS) and 2100MHz

GSM bands not supported in the USA, but active in other countries: 900 and 1800.

Internationally, you may need the 900 and 1800 bands, especially in Europe and Asia (many south American countries work on the 1900 GSM band). If you have a “quad-band” phone it will work everywhere.

Sometimes these bands are called by acronyms instead of frequencies:
UMTS: 2100
PCS: 1900
DCS: 1800
AWS: 1700 (or simply AWS)

The advanced wireless system is a new block of bandwidth that has been opened up for use in the USA. AWS Overlaps with UMTS and DCS (1700/1800 and 2100) The convention appears to be labeling AWS with the number 1700, because 1800 and 2100 are already used to indicate other things.