The Subscriber Identity Module is a smartcard that is necessary to make use of a mobile phone. The SIM is the key used to identify and authenticate the mobile subscriber. On the SIM is also memory available for personalised data, such as a telephone book and messages.
The subscriber is identified with an IMSI, International Mobile Subscriber Identity, and a telephone number.
The SIM made a clear seperation between a mobile phone and a subscriber possible. The subscriber can make use of any mobile phone under his own account if the SIM card is put in the phone.
There are two different sizes available, a credit card size and a small size that is of the size of he chip that is on most credit cards. Nearly all phones use the small version.
The use of a SIM can be guarded with a PIN code from 4 to 8 digits. If 4 times the wrong PIN code is typed, the SIM will be blocked. To unblock the SIM the PUK (PIN Unblocking Key) is needed. This is an 8 digit code that is known by the authorised user and given by the service provider.
Nowadays, there is also an embedded SIM (eSIM) available.