SIM is short for Subscriber Identity Module. A SIM card is a portable memory chip (like a mini hard disk) that automatically activates the cellular/mobile phone into which it is inserted. They are used with carriers that operate on the Global System for Mobile Communication (GSM) network. Typical international frequencies for GSM cell phones operate on two or three out of four different frequency band types: 850 MHz, 900 MHz, 1800 MHz and 1900 MHz.
SIM cards make it easy to switch to a new network by Simply swapping the SIM in an unlocked phone with a local network SIM card. The SIM holds personal identity information, your phone number, phone book, text messages and other identification data.