A SIM card, or subscriber identity module, is a small smart card that stores data for subscribers of GSM cellular phones. It contains a unique identification number, phone number, and other data that links to the consumer. SIM cards are necessary for a GSM phone to work, as they allow the phone to access the local mobile network to place or receive calls, send texts, and use mobile data. Without a SIM card, some phones would not be able to make calls, connect to the internet, or send SMS messages. SIM cards contain a variety of information, such as who is paying for the access and that you have permission to use those features such as placing a call or sending a text. Since SIM cards hold the subscribers information, they come in handy when the battery on your phone is low or needs to be recharged. You can take out the SIM card and place it into another phone and send a text or make another call. Since you have the SIM card in the phone, any calls or texts will show your phone number and that it is coming from you. SIM cards are removable and transferable to other phones, and they come in different sizes. They can be cut down to smaller sizes, so you don’t have to purchase a new card when you switch to a different phone.