Subscriber Identity Module (SIM), popularly known as a SIM card, is a minute processor that holds essential information related to the user's telecommunication service.

At its core, SIM card acts as a copyright to accessing communication networks, allowing mobile phone owners to partake in telecommunication services. Furthermore, it also aids in storing important phone numbers, messages, and even network-specific data.

There are commonly three sorts of SIM cards present in the market: the standard size, micro, and nano size. These variants vary in size, but produce the same functionality.

Each SIM card comes characterized by unique International Mobile Subscriber Identity (IMSI) number, which is consists of a fixed length numeric code. This distincts each subscriber within the network.

Moreover, every SIM card is programmed with unique Ki key, which serves for securing communication over the telecom network.

Switching to these SIM cards' functions, they hold details such as a user's phone number, service details, and security data. Apart from these, they allow additional services such as call advancing, call barring, and voice mail.

In conclusion, it's clear that SIM cards are crucial parts of website our mobile phones. They are like the identity proofs for our phones, allowing them function correctly. Knowing your SIM information assists in resolving any issues related to your telecommunication service.

Additionally, understanding your SIM card can also help in making decisions about upgrading your phone, moving network service, and even in some cases traveling internationally.