SIM card

(pronounced: simm card)

A SIM card is a portable memory chip used in some models of cell phones. The SIM card makes it easy to switch to a new phone by simply sliding the SIM out of the old phone and into the new one. The SIM stores personal identity information, cell phone numbers, text messages and other data. It can be thought of as a mini hard disk that automatically activates the phone into which it is inserted. 

An eSIM, or embedded Subscriber Identity Module, is a digital SIM card that's permanently built into a device, eliminating the need for a physical SIM card. It allows users to switch between carriers, manage multiple SIM profiles, and activate new plans without swapping physical cards. 

Historical perspective: SIM cards are used with carriers that operate on the Global System for Mobile Communication (GSM) network. The competing network is Code Division Multiple Access (CDMA), a technology created by U.S. company Qualcomm. As of fall 2005, CDMA cell phones and CDMA carriers do not support SIM cards in most parts of the world, though this is changing. A CDMA SIM card called the R-UIM (Re-Useable Identification Module) was made available in China in 2002, and will eventually be available worldwide. Expectations for the future include a cell phone market that supports both SIM (GSM) and R-UIM (CDMA) cards by default.