What exactly is a SIM card? You know, the little piece of plastic found in the metal pullout on the right side of your iPhone or cellular iPad? The short answer: it's a Subscriber Identity Module, a small circuit board in most modern phones to communicate with your carrier. Practically speaking, it is a middleman between two pieces of hardware: the phone's baseband chip, and your carrier's cell towers, allowing the two to communicate.

SIM cards allow you to travel between phones with your phone number and data plans; when you upgrade to a new iPhone, the SIM card travels (as long as it is the same size). But while SIM cards themselves may be portable, they only work in certain phones. We'll explain why in a bit.

A SIM card is internationally identified by its Integrated circuit card identifier (ICC-ID), which is engraved on the body of the card. It is also identified by the carrier with its International mobile subscriber identity (IMSI). Essentially, these two numbers tell the carrier that your phone is allowed to operate on its network and, once connected, should be billed for certain features. Beyond identification, SIM cards have several other functions.

What is a SIM card?

SIM cards have evolved a lot over the years. While they have maintained a relative thickness of just under 1mm, their surface area has steadily decreased, from the credit card-sized plates used in the earliest cellphones to the nanoSIMs of today's devices.

The nanoSIM is the SIM card's fourth size standard since its inception. Designated as 4FF, or fourth form factor, it measures 12.3mm x 8.8mm x 0.67mm, a reduction of over 42 times from the SIM's inception. Most people, however, are more familiar with the 2FF SIM card, known more readily as the Regular SIM card. At 25mm x 15mm x 0.76, it is 3.4 times larger than today's current SIM cards, which offer the same features in a much more compact package. Some manufacturers still choose to use the microSIM format which, at 15mm x 12mm, is only slightly larger than the nanoSIM.

Many carriers offer branded SIM cards with cutouts for all three modern sizes, so users can choose which version they want to insert depending on their device. There are also adapters so nanoSIM cards can fit into slots meant for microSIM or Regular SIM cards.

How does a SIM card work?

Often, a SIM card is provided with the purchase of a phone by your carrier and it is used to store data about your account. Because it has a small amount of memory and a very low-powered processor, the SIM card not only enables communication between the phone and its carrier, but stores information such as phone numbers, security data and more. In recent years, carriers have begun using specialized SIM cards with so-called Secure Elements to store credit card credentials in order to facilitate mobile payments.

Bonus Tip: SIM cards are transferrable. If your iPhone runs out of power and you desperately need to make a call or connect to the internet, you can just swap the SIM into another iPhone and use it with your minutes and data bucket.

What are the benefits of a SIM card?

SIM cards are the unseen magicians of today's smartphones. They make connecting to networks and switching phones as easy as removing a small metal tray.

If you buy a new phone, you can simply insert your existing SIM and keep on using your existing service as long as the new phone isn't locked to a different carrier. Likewise if you travel internationally you can just buy a SIM card from a local carrier — as long as your phone isn't locked to a carrier.

What are the drawbacks of a SIM card?

SIM cards are fairly simple. They have practically no function without an accompanying smartphone and are increasingly reliant on third-party apps to address functionality, such as connecting to multiple networks or using more than one phone number.

And because they are simple pieces of plastic, their benefit is largely subject to the will of the carrier. If you buy a phone that is locked to one carrier, putting a different carrier's SIM requires an unlock code, which is often expensive to obtain and confusing to enter.

Typically the SIM lock is in exchange for a subsidized phone (so you can't buy a cheap phone on one carrier then switch over an use another before you've paid back the subsidy over the life of your contract).

These days, Apple sells the iPhone SIM-unlocked in most countries, including the U.S., Canada and the U.K. While customers have to pay more upfront for an unlocked version of the iPhone, they can use the phone with any compatible SIM card, as long as the phone works with that carrier's network. The latest iPhone, the 6s, supports 23 LTE bands, which comprises most of the world's carriers.

What is Apple SIM?

In 2014, alongside the iPad Air 2 and iPad mini 4, Apple unveiled Apple SIM, a way for the company to assert some control over its devices' connection to various carriers.

By working with select providers in the U.S. and U.K., Apple SIM allows iPad owners to connect to whichever network they want — provided that telco has worked with Apple. In the U.S., those carriers include Sprint and T-Mobile; in the U.K., EE. The beauty of Apple SIM is its ability to interface with multiple networks; when visiting foreign countries, partners GigSky, AlwaysOnline Wireless and au work with local providers to offer per-day or per-megabyte bundles.

Most recently, with the 9.7-inch iPad Pro, Apple began embedding its Apple SIM in its tablet line, allowing the use of both Apple's partner networks as well as local carriers, such as AT&T and Verizon in the U.S.