SIM, subscriber identity module, is a smart chip for mobile phones which is used in all GSM and 3G networks. In 3G it is called USIM. It's also used in some version of CDMA. The SIM is identified on their network by their unique IMSI (International Mobile Subscriber Identity) number.
 

Description

SIM is behaving like a mini computer with its own operating system and storage. It can add delete and control information in its memory and both send and receive data. SIM is available in two sizes one is 85.60 mm × 53.98 mm x 0.76 mm while the second which is most widely used has a width of 25 mm, a height of 15 mm, and a thickness of 0.76 mm. The basic functionality of SIM is to give access of network to the mobile phone. It authenticates the user to the network and check that they billed correctly with respect to the airtime they used.

Functions

SIM allows the storage of messages, personal phone directory, and give the roaming facility. Through SIM the user can access the news, games, and lots more. Through SIM use can connect internet via GPRS. Through SIM one can send SMS, MMS to their friends. But these all things depends upon the type of service company is providing and the mobile phone customer is using. 

SIM also stores information of its current local area identity (LAI). When user changes his area then the SIM updates the location and send the information to the network about the new location. When the phone turn off and back on again it will check the LAI and send update to the network with respect to it.

SIM is available in different storage sizes the biggest being using is 128 K while the mostly widely used version has storage space of 64k.