A cellphone SIM card contains the mobile account information that allows the phone to attach to a mobile network. SIM cards can store a limited amount of information and data.
A SIM card helps cellphone carriers identify the user of the phone and the account holder of the cell phone service. SIM cards can often be moved from one cellphone to another without having to contact the mobile network provider. Text messages and phone numbers are just some of the data that SIM cards store. They are about the size of a postage stamp and run at a speed of about 5 megahertz.