The Full form of SIM is Subscriber Identity Module or Subscriber Identification Module. SIM is an integrated circuit that securely stores the International Mobile Subscriber Identity (IMSI). It is a portable memory chip that makes you able to make phone call or send text messages across the world where subscriber’s network is available. SIM is a smart card that stores data for GSM cellular telephone subscribers. Such data includes user identity, phone number and location, network authorization data, personal security keys, address book and stored text messages. A SIM also contains its unique serial number (ICCID), International Mobile Subscriber Identity (IMSI), security information, ciphering information, temporary information related to the local network, etc. Security features include authentication and encryptions to protect data and prevent eavesdropping. SIM was almost of the size of a credit card when it was first invented. But the newest standard of a SIM contains a size of 15mm by 12mm. Now a days, some smart phones use micro and nano SIM cards which are different in sizes. There is a difference between GSM SIM card and CDMA SIM card. CDMA phones generally don’t use SIM cards. Instead, all the information is saved in handset itself. It may be less convenient for the users because user can’t change the service provider for the same handset.