A SIM card (Subscriber Identity Module) is a small, removable card used in mobile devices like smartphones and tablets. It stores essential information, such as the user's phone number, network details, and contacts, allowing the device to connect to a mobile network. When you insert a SIM card into a device, it enables you to make calls, send texts, and access mobile data. Different types of SIM cards, such as nano SIM, micro SIM, and standard SIM, vary in size but serve the same purpose of connecting the device to a cellular network.