The SIM card is a tiny, rectangular card found in all GSM cell phones (think T-Mobile and Cingular) that acts as an ID card for your handset. Usually located behind the battery, a SIM card stores your phone number and is needed to activate a mobile for use. If the card is taken out of the handset, it stops working, and it is no longer associated with your phone number. The advantage of a SIM card is multifold (sorry, Verizon and Sprint folks). Most importantly, you can switch your phone number from one mobile to another simply by swapping out your SIM card. As long as both phones are for the same carrier, the switch is fast and painless. And on a related note, if your handset is unlocked (meaning it's not locked to one carrier only) you can buy prepaid SIM cards when traveling so that your mobile will always have a local number for your destination. And finally, SIM cards can store up to 250 contacts, which makes for easy transfers of your phone book.