From Forum Nokia Wiki

• SIM stands for Subscriber Identity Module, which is a small smart card which contains both programming and information.

• The A3 and A8 algorithms are implemented in the Subscriber Identity Module (SIM).

• Subscriber information, such as the IMSI (International Mobile Subscriber Identity), is stored in the Subscriber Identity Module (SIM).

• The Subscriber Identity Module (SIM) can be used to store user-defined information such as phonebook entries.

• One of the advantages of the GSM architecture is that the SIM may be moved from one Mobile Station to another. This makes upgrades very simple for the GSM telephone user.