AWS is a set of spectrum at 1710-1755 and 2110-2155 MHz. One is used for uplink the other for downlink. The spectrum is split into 6 different blocks (A-F).
For more info this is a good read http://www.phonescoop.com/articles/article.php?a=99&p=1492
Verizon, Sprint, and US Cellular don't...