Subscriber Identity Module, also called a "SIM card"
Phones using the GSM mobile standard require SIM cards. The SIM card contains information about the subscriber (e.g. your cellular phone number) as well as the provider (e.g. AT&T, T-Mobile). Pre-paid SIM cards are often a cost-effective option for international travel, if your phone is unlocked. A locked phone can only use SIM cards issued by a specific mobile provider.