SIM Card – SIM is short for Subscriber Identity Module. SIM cards are small removable smart cards that are used in many cell phones to store data like a customer's mobile phone number, the mobile phone company they use, and address book information.
SIM cards allow users to easily transport their phone numbers and contacts to new phones simply by removing the card from the old phone and inserting it in the new one.
Not all phones have SIM cards and some that have them do not allow users to remove them.
Some phones have what is called a SIM lock, a feature that uses the SIM to tie the mobile phone to a specific phone company (usually the one who the user bought the phone from originally). This is done in part because phone companies often subsidize the purchase price of phones, and because they want to lock customers into using their service. Phones without SIM locks are called unlocked.
In many countries, it's possible to purchase an unlocked phone for the full retail price or to pay a small fee to carriers after a defined contract length to unlock the phone. Users can also unlock phones via software hacks.
The iPhone, iPhone 3G, and iPhone 3GS used the Mini-SIM card format. With the iPhone 4, Apple began using the even smaller microSIM card.
Related: More on the iPhone 4 SIM
The SIM card on iPhones can be removed by inserting a bent paperclip into the tiny hole in the SIM tray. On the iPhone 4, the SIM tray is on the phone's right side. On other models, it's on the top of the phone.
Unlike some other mobile phones, the iPhone's SIM cannot be used to store information; The user cannot directly interact with the data on the iPhone's SIM.