A SIM (Subscriber Identification Module) is the smart card included in every cell phone and smart phone of the GSM family of networks. It's a sophisticated computer chip embedded in a small piece of plastic about the size of a thumbnail and placed behind the battery.
The SIM card identifies your plan and services to your operator. It allows you to receive wireless phone service when you are within coverage of your operator's network. In addition, the SIM is an essential part of the GSM system that enables your phone to work internationally in countries where your cell phone provider allows it. It is used by hundreds of operators in more than 200 countries around the world.
The SIM securely stores the contacts in your phone book and other personal information. Finally, it protects your digital life by preventing anyone from running up your phone bill with fraudulent calls and by securing your calls with encryption so they cannot be eavesdropped.