What is a SIM card?

The SIM (Subscriber Identification Module or Subscriber Identity Module) card - a.k.a. "smart card" - holds all of a subscriber's personal information and phone settings. In essence, it is the subscriber's authorization to use the network. It also holds the phone number, personal security key and other data necessary for the handset to function. The card can be switched from phone to phone, letting the new phone receive all calls to the subscriber's number.