A SIM card is a smart card that is used in mobile phones, to identify the client. SIM stands for subscriber identity module. A SIM card has a microchip, and its use is protected by a PIN. When the phone is powered on, an international mobile subscriber identity number (IMSI) is broadcast. The microchip is also needed for some encryption and decryption. The SIM card, and the device it is use in (the mobile phone) are often referred to as Mobile station.
SIM cards come in different sizes. Full-size SIM cards, which are about the same size as a credit card, are not used in phones anymore. Most smartphones use either micro SIM or nano SIM cards, but older ones use mini SIM.