The SIM (Subscriber identity module) card is a card used in GSM, CDMA mobile phones that stores the account data for cellular telephone user. It is an Integrated circuit chip which stores the International mobile subscribers identity (IMSI) number. This IMSI is unique for every SIM card.
SIM cards are capable of offering 2G, 3G and 4G LTE data services. A SIM card stores the data like user identity (ID), account balance & plan details, geo location, phone number, personal SIM number, network operator (carrier) authorization, security key, text SMS messages and phone contact list.
Normal Full-size SIM card can be trimmed to fit in micro, mini or nano SIM type USB modem dongle or phones (android, phone). There are three operating voltages for SIM cards: 5V, 1.8V and 3V ((ISO/IEC 7816-3 classes A, B and C respectively)