Subscriber Identity Module, usually known as a SIM card, serves as a tinny smart card that carries crucial details about the user's subscription.

At its core, SIM card works as a key to accessing a cellular network, here allowing mobile phone owners to send and receive calls. Moreover, it also aids in retaining crucial personal contacts, messages, and even in some cases data related to the cellular network.

There are basically three kinds of SIM cards available in the market: standard, micro, and nano size. Each of these versions vary in physical dimensions, but offer similar functionality.

Each SIM card comes identified by unique International Mobile Subscriber Identity (IMSI) number, which is consists of a 15-digit numeric sequence. This differentiates each user within the network.

Moreover, each SIM card comes configured with unique Authentication Key (Ki), which acts for guaranteeing data exchange over the service provider's network.

Talking about SIM cards' functions, they retain data like a user's contacts, service information, and security data. Apart from these, they allow services including call forwarding, call barring, and voicemail.

In summary, we see that SIM cards are essential components of our mobile phones. They serve as the identity cards for our mobile networks, making them work correctly. Knowing your SIM information helps in solving issues related to your mobile network.

Furthermore, having knowledge about your SIM card can also help in streamlining decisions about upgrading your phone, changing service plans, and even travelling abroad.