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 authenticate the user and enable communication services like calls and text messages. SIM cards come in different sizes, including nano, micro, and standard formats, to fit various devices. They can be easily swapped between phones, allowing users to change networks or keep their numbers when upgrading devices. Overall, SIM cards are crucial for mobile connectivity.