Subscriber Identity Module.
A small "smart card" used in GSM and LTE phones.
The SIM card contains a microchip that stores data that identifies the user to the network. In a sense, the SIM card is what links a phone to its phone number. The data is also used to encrypt voice and data transmissions, making it difficult to intercept calls and data using nearby radio equipment.
The SIM can also store limited phone book information - phone numbers and associated names.
The SIM, typically located under the battery, but sometimes in a tray on the side, can easily be removed and placed in another phone. This will cause the new phone to instantly operate using the subscriber's existing phone number and account.
SIM cards come in several sizes, including micro-SIM and nano-SIM.
All GSM phones and most LTE and iDEN phones use SIM cards. CDMA phones generally do not use SIM cards. The information is instead programmed directly into the phone. However, some CDMA phones for China and Latin America use a very similar type of card called R-UIM.
WCDMA / UMTS (3G) and most LTE (4G) phones use an technically upgraded version of SIM cards called USIM cards: