SIM card

A subscriber identity module or subscriber identification module (SIM) 

An integrated circuit that is intended to securely store the international mobile subscriber identity (IMSI) number and its related key, used to identify and authenticate subscribers on mobile telephony devices.

Full-size SIM (1FF, 1st form factor)

Mini-SIM (2FF)

Micro-SIM (3FF)

The use of SIM cards is mandatory in GSM devices.

CDMA-based devices originally did not use a removable card, and the service for these phones bound to a unique identifier contained in the handset itself. This is most prevalent in operators in the Americas.

SIM CARD PINS 

1. VCC (Power supply)
2. RST (Reset)
3. CLK (clock)
4. D+ (USB Inter-chip)
5. GND (ground)
6. VPP (voltage programming power)
7. I/O (input/output)
8. D (USB inter-chip)