A SIM card, also called a subscriber identity module or subscriber identification module, is a small memory card that contains unique information that identifies it to a specific mobile network. This card allows subscribers to use their mobile devices to receive calls, send SMS messages, or connect to mobile internet services. SIM cards have a seventeen-digit code that designates its country code of origin, the system carrier (such as AT&T or Verizon), and a unique user ID.
SIM cards were introduced all the way back in 1991. The first SIM card was about the size of a 1FF full size smart card such as a credit card. Since then, there have been several updates and iterations, making them smaller and smaller.
This evolution closely aligns with the history of the GSM (Global System of Mobile Communications) mobile network. Developed to describe protocols for second generation (2G) cellular networks, the GSM today stands as the default global standard for mobile communications.
SIM cards are a crucial part of GSM networks, storing user information required for authentication. Ultimately, this allows both your phone to connect with a GSM network and GSM networks to track your phone usage (specifically, your minutes, texts, data usage, etc), giving carriers the information to send you an accurate phone bill.
GSM is the standard network in Europe, Asia, Africa, and most other places. Interestingly, besides GSM, the United States has an additional network called CDMA (Code Division Multiple Access) that is used by carriers such as Verizon and Sprint. Phones on the GSM or CDMA network can use any carrier’s network, lowering the costs for the carriers and its users.