SIM card is the subscriber identification card of digital cellular mobile phone. Its full name is "subscriber identification module". In the GSM digital mobile phone system, the SIM card plays an extremely important role. It is not only an important part of the mobile phone, but also an identity card for every GSM mobile phone user. Therefore, the SIM card realizes the concept of recognizing people and not recognizing machines.

The third category is related business codes, such as personal identification code (PIN), unlock code (PUK), billing rate, etc. The fourth category is the telephone directory. Almost all user information is stored in the SIM card, so the SIM card is also called "user information identification card". SIM card can be divided into big card and small card. The small card is called "plug-in SIM card", which is the chip on the big card.