A SIM (subscriber identity module) card is a small memory chip that is used in some mobile phones. The SIM card is used in order to make it easy to switch to new phones. All one has to do is to remove the SIM card from the existing phone and then putting it in the new mobile phone. The SIM card's memory is able to hold the personal identification information, cell phone numbers, phone book, text messages and other phone-related data. A SIM card can be seen as a small hard drive that can instantaneously activate a phone when it is inserted.
Using a Sim card can be quite convenient because of its swapping feature. If a person's phone runs out of battery, he can just borrow another person's mobile phone, insert his SIM card, and he can already use his own phone number to call and receive phone calls from his telephone number. What's more, even if he uses other people's phones, the person who lent the phone won't be charged anything.It is the SIM card owner who will be charged for the phone calls. The phone carrier will see the phone call as still coming from the person's SIM card.
With a SIM card, it is even possible for a person to acquire many phones, and just swap the SIM card to whichever phone he wants to use. In some countries, using a SIM card also means that the person can switch mobile phone carriers and yet still retain his old phone number.