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, a special number called 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.