A SIM basically provides a means through which a device can connect to a mobile network, provided by a mobile network operator (MNO). Further, while allowing connectivity is essential, the SIM also enables registration and authentication – just because a device can connect to the mobile network, it doesn’t mean that it should be allowed to do so.
The SIM handles this process. It can be a physical device that you can insert or remove – just like the ones you use in your normal mobile phone – or it can be an integrated component, as we shall see. An IoT SIM is simply one that is optimised to support data services, which is what most devices need.