A SIM (Subscriber Identification Module) is the smart card that is included in every cell 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. It's like a small, highly secure computer with its own operating system. It uses versions of many of the standard computer languages such as Java & HTML. The gold shape creates a contact between the chip inside the SIM and the phone to allow communicated between the 2 devices.
The SIM is present in cell phones from companies such as AT&T and T-Mobile in the U.S and many others worldwide. They are also present in smart phones like the BlackBerry and iPhone. Other cell phone providers carry the SIM only for their international roaming phones. The SIM card identifies you to your cell phone carrier (like AT&T for example) so they know your plan and services. The SIM allows you to receive wireless phone service whenever you are within coverage of your cell phone provider'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 cell phone companies in more than 200 countries around the world The SIM also 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 that they cannot be eavesdropped.