SIM (Subscriber Identity Module) cards are small chips used in mobile phones to store data such as the mobile phone number, subscriber information, and the mobile phone network's security encryption key. When a mobile phone is turned on, the SIM card connects to the mobile phone network and verifies the subscriber's information, allowing the phone to make and receive calls and send and receive data. The SIM card can be removed from one phone and inserted into another phone, allowing the subscriber to switch phones without having to change their mobile phone number or subscribe to a different network.