A SIM card, or Subscriber Identity Module, is a portable memory chip for GSM cell phones. SIM cards are used to store personal information, phone numbers and addresses, as well as information like text messages. SIM card technology allows GSM customers to quickly transfer their entire account, address book and all, to a different GSM phone by simply switching the card. An example of what a SIM card looks like can be found below. SIM cards will vary in color and logo but are uniform in shape.