A SIM, or Subscriber Identity Module, is a small 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 card allows users to make calls, send texts, and access mobile data. SIM cards come in different sizes, including nano SIM, micro SIM, and standard SIM, to fit various devices. They can be easily swapped between phones, enabling users to change networks or devices while keeping their phone number and data intact.