GSM is the system that uses a SIM card. The card is the phone number and information needed for the carrier. Most of the world uses GSM. When you are in a different carriers area you pull the SIM card for the previous carrier and insert a local one. Easy.

CDMA doesn't use a SIM card. The phone is tied to the carrier. Verizon and Sprint are the two biggest US carriers that use CDMA so if you're with one of those carriers you need a special phone that is tied to that carrier. Other carriers such as AT&T and T-Mobile use GSM and a phone that uses a SIM card is needed. You can use the same phone for either carrier.

I'd rather have a GSM phone as that's what most of the world is. Since my son travels he uses only GSM except for Japan where he has a special phone for that country. The Sony USB modem uses GSM technology. If one for Sprint is purchased it can only be used for Sprint.

Dave asked about Sprint so I included a link to a Sprint USB modem. Again, I think it's better to work out a GSM system and forget about CDMA (Verizon & Sprint) if you are someone who travels.

The knowledge I have is only from my own experience traveling in Guatemala, Panama, Belize and other Central American countries. We use an Iphone 3gs when traveling. The Iphone is jailbroken and unlocked. Wherever we are we just pop in a SIM and we have service. In Guatemala we needed a SIM for the Iphone and then we had to purchase a 'TIGO' USB stick for the broadband internet.

Along with the GSM & CDMA is the system for data. It goes hand-in-hand with GSM (SIM) & CDMA (no SIM). A carrier that uses the GSM system has it's own data system that works with it, generally GPRS or EDGE. A CDMA carrier generally uses CDMA2000.

Anyone feel free to correct me. I only know a little of this stuff and I know it's a bit weak or perhaps ill informed.