What are SIM cards and how do they work?
A SIM card or Subscriber Identity Module is a portable memory chip used in GSM cell phones. It is about the size of your thumbnail and is usually installed just under a cell phones battery. It not only stores information like your phone book but allows your service provider to recognize your cell phone.
SIM cards are ONLY used by service providers that operate on the Global System for Mobile Communications (GSM) network. The competing network is Code Division Multiple Access (CDMA), a US technology that does NOT use SIM cards.
A cell phone that uses a SIM card provides a big advantage to international travelers. Simply take your International Dual or Tri Band GSM phone with you and buy a local SIM card and Pre-Paid Calling Time or "minutes" at your destination. For example, a traveler from the U.S. going to Thailand can purchase a Thai SIM card when he arrives and be ready to receive and make calls almost immediately. Additionally, if he should decide to visit Saigon on his way home, he just purchases another SIM card in Saigon and keeps using the same phone.
American cell phone providers expect you to change cell phones if you want to use another network; in the rest of the world you just change SIM cards.