A Subscriber Identity Module, commonly known as a SIM card, is a small card inserted into mobile devices like smartphones and tablets. It securely stores information about the user's identity, including their phone number and network details. This allows the device to connect to a mobile network, enabling calls, texts, and data services. The SIM card also plays a crucial role in security by encrypting communications and authenticating the user to the network. There are different types of SIM cards, such as nano SIM, micro SIM, and standard SIM, which vary in size but serve the same fundamental purpose.