Subscriber Identity Module

A subscriber identity module is a smart card which securely stores the key identifying a mobile phone service subscriber, as well as subscription information, preferences and text messages. The equivalent of a SIM in UMTS is a Universal Subscriber Identity Module (USIM).
The SIM stores network state information such as its current location area identity (LAI). If the handset is turned off and back on again it will take data off the SIM and search for the LAI it was in. This saves time by avoiding having to search the whole list of frequencies that the telephone normally would. Each SIM is uniquely identified by its ICCID [International Circuit Card ID].
SIM cards identify users uniquely by holding an IMSI.