A Subscriber Identity Module (SIM) card is a small, removable card in mobile phones and devices that stores user information, such as phone numbers and carrier details. The SIM card allows the device to connect to the mobile network, enabling voice calls, text messaging, and data services. It securely holds unique identifiers like the IMSI (International Mobile Subscriber Identity) and authentication keys, allowing users to switch between devices by simply transferring the SIM card. SIM cards are essential for mobile communication and have evolved to include smaller versions like microSIM and nanoSIM.