Subscriber Identity Module (SIM) is a removable smart card for mobile phones. SIM cards securely store the service-subscriber key (IMSI) used to identify a GSM subscriber. The SIM card allows users to change phones by simply removing the SIM card from one mobile phone and inserting it into another mobile phone.
The traditional SIM card issued by a mobile operator contains a single IMSI for the issuing operator. The IMSI is then used to authorize access to mobile networks, whether the home network or another network the device has roamed.
A SIM card can typically contain, the telephone number of the subscriber, encoded network identification details, the PIN and other user data such as the phone book. A user's SIM card can be moved from phone to phone as it contains all the key information required to activate the phone.