Frequencies by Carrier
So now we know what GSM and CDMA means, so here's a quick list of the frequencies you'll see on most U.S. based networks.
GSM: 850Mhz (AT&T) and 1900Mhz (T-Mobile) - Of course there are other regional networks you'll have to find out which one of those two frequencies they run on.
GSM: 900Mhz, 1800Mhz - These frequencies are used in several international regions and are included on Tri-Band and Quad-Band GSM devices.
This brings us to an important point. Devices usually carry more than one frequency. The most popular options are Tri-Band GSM devices (900/1800/1900Mhz frequencies) and Quad-Band GSM devices (850/900/1800/1900Mhz frequencies).
In terms of CDMA the main frequencies of importance are the 850/1900Mhz frequencies which you'll find in use at Verizon, Sprint, Alltel, etc.
Simply put, you'll want to find out which frequency your cellular carrier implements and make your choices from that point.