GSM stands for Global System Mobile. When this technology is available on a BlackBerry mobile phone, a removable SIM card (Subscriber Identity Module card) is installed inside the phone (normally beneath the phone’s battery). With GSM technology, SIM cards can be removed, switched to another GSM phone and another one can even be used it its place. For example, if you’re traveling to another country where GSM technology is available, instead of incurring roaming charges, many BlackBerry users will, instead, buy a prepaid SIM card for use in the country that they’re visiting and make all of their local calls on that card. By the same token, GSM technology is also used in several other countries which also makes GSM more desirable to some as it gives them a wider service area (even if they are roaming in these other areas).
Also, because of the removable SIM card, GSM phones are easier to upgrade. All it takes is buying the GSM enabled phone of your choice and inserting the new SIM card. This process doesn’t involve contacting the carrier to make a switch.