What are Sim Cards of the mobiles?

SIM (  subscriber identity module or subscriber identification module ) is the card which is used in the mobile. A Sim  is an integrated circuit  which stores the service subscriber key  ( IMSI )   which is used to identify  a subscriber who has mobile connections like computers and mobile phones.
It is a removable sim card  and can be transferred from one phone to another.

First the sim cards were made like the present day credit cards, but  the development of smaller sized mobile phones prompted   smaller sized sim cards development. These were mini sim cards and were of the same thickness of the older  and bigger simcards  but the length and width were reduced to  25 mm × 15 mm.

Sim Cards store informations which are network specific  and used to authenticate  as well as identify their subscribers on the network. The most important information  stored in the sim cards are ICCID,IMSI, Authentication keyLocal area identity (LAI) and an  Operator-Specific Emergency Number.
The SIM  can also store other specific data such as the SMSC (Short Message Service Center) number, Service Dialing Numbers (SDN), Service Provider Name (SPN), Advice-Of-Charge parameters and Value Added Service (VAS) applications.