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, contacts, and network authorization data, allowing the device to communicate with the network provider. When you insert a SIM card into a phone, it enables you to make calls, send texts, and access mobile data. Different types of SIM cards, like nano SIM and micro SIM, vary in size but serve the same purpose of linking your device to a specific mobile network.