GSM (Global Services for Mobile) is the second most common type of digital mobile phone used today, right after CDMA. GSM is being used primarily inside and outside of the United States. GSM outsmarts all other types of wireless technology due to one valuable feature. The fact that your phone is not tied to your GSM service enables cell phone users to simply replace one SIM card with another SIM from another service provider, without having to switch handsets. You may want to buy an unlocked GSM cell phone in order to use it with every company's SIM.