Your question may be what is a SIM card and what does it do?  A SIM card is a “Subscriber Identity Module” also you may hear to it referred to as a “smart chip” by some people.  Generally SIM card is the correct way to identify it as, as calling it a smart chip can be confused with other types of memory or chips/memory cards.  So what does it do?  The easiest way to think of it is that the SIM card identifies your GSM phone with the tower and your phone number/account.  You can put your SIM card in any unlocked GSM phone and it will allow you to use your service and phone number on your network.  What else is stored on your SIM card besides the information to identify your phone to the GSM network/system you are using?  You can save your phone numbers to your SIM card so that when you transfer it to other phones you will already have your phone book in the new phone.  Also depending on the phone, some will store your text messages to your SIM card.  I get a lot of questions asking about pictures being stored to the SIM card, which they are not.  Generally SIM cards KB sizes are 32 or 64kb. 

ATT Wireless has started using 64kb SIM cards for their new phones and require them for their 3g devices.  Most new ATT 3g devices will not work properly with an older 32kb SIM card and will have some issues, including accessing the Internet and voice mail systems.

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