Subscriber Identity Module (SIM)

A small, stamp sized smart card that is used in GSM phone. This small computer chip holds a unique ID (known as an IMSI) that the network operator uses to identify the user account to the network. The card also handles authentication and provides data storage for basic user data and network information. The data storage is also used to encrypt voice and data transmissions, making it nearly impossible to listen in on calls.

Storing between 16k and 64k worth of data, it also has storage space for SMS text messages and address book information. The SIM can easily be transferred to other handsets allowing the subscriber to change the mobile phone along with existing text messages and contact details.

AMPS and CDMA phones do not use SIM cards as information is sent directly into the phone.