A SIM card (Subscriber Identity Module) is a small, removable card used in mobile devices to connect to a cellular network. It stores essential information, such as the user's phone number, network details, and contacts. This allows the device to communicate with the network and enables services like calling, texting, and mobile data. When you insert a SIM card into a phone, it authenticates your device with the carrier, allowing you to access their services. Different types of SIM cards, such as nano, micro, and standard, vary in size but serve the same purpose of connecting your device to the network.