Jason Cipriani/CNET

Samsung's Galaxy S20 and Galaxy Z Flip are joining the iPhone 11, Pixel 4, Galaxy Fold and the new, foldable Motorola Razr as the small handful of phones that use a new technology called eSIM. What that means depends on the phone. For some, like the Razr, you can ditch your plastic SIM card altogether. For others, like the iPhone 11 and Galaxy S20, you can have two phone numbers assigned to one phone.

A SIM card is the essential ingredient in connecting your phone to a cellular network, making calls and sending texts using your phone number. An eSIM does the same thing, but instead of a plastic chip you can remove from your phone, it's part of your phone's internal components.

Admittedly, an eSIM isn't nearly as exciting as a foldable screen or a camera that can take pictures in the dark, but it's a vital part of our phones. Without a SIM card, our phones can't connect to our wireless carriers, and may as well be iPods. Alongside technological advances when it comes to displays, the SIM card is changing -- and it's for the better.

What exactly is an eSIM and why should you care? Glad you asked. Here's everything you need to know about eSIM technology and why it will start to matter.

Now playing: Watch this: Hands-on with the Galaxy S20, S20 Plus and S20 Ultra

What is an eSIM?

An eSIM -- that stands for "embedded subscriber identity module" -- is a small electronic chip inside a phone that serves the same function as the small plastic SIM cards we've used for years. Without a SIM card, your phone doesn't know which carrier to connect to or your phone number, and your carrier doesn't know which phone to provide service to.

Jason Cipriani/CNET

What are the benefits of an eSIM?

Unlike carrier-locked SIM cards that restrict you from switching between carriers, an eSIM can be programmed and reprogrammed as needed, allowing you to hop between carriers or cancel and start service without having to get a new SIM card each time.

Without a removable SIM card tray and the precious space that it requires, phone-makers will be able to squeeze larger batteries inside a phone's housing or use the extra space for additional cameras or new features. Of course, no SIM card slot likely means no microSD card slot for adding additional storage, a change that phone-makers are also migrating toward.

The eSIM is still young, and as such the process for things like activation and support from carriers and phone-makers will need to be streamlined (more on both of those aspects below), but one thing is clear: One day soon, we'll be able to instantly switch between carriers with just a few taps on our phone's display.

Juan Garzon/CNET

Which phones use an eSIM now?

There aren't many phones that use eSIMs right now.

Jason Cipriani/CNET

Why have an eSIM and a physical SIM?

Apple's iPhones that have two SIM cards -- an eSIM and a physical card -- can connect to two different carriers and phone numbers at the same time. So you can have your personal and work number on one iPhone, even if you have T-Mobile and Verizon, respectively, as providers. This is possible on an iPhone because it's unlocked and supports multiple carriers.

Phones with eSIMs are also handy for frequent travelers: You can use the eSIM to sign up for a data plan overseas and save yourself from roaming fees. And, if you set up an eSIM through a provider like GigSky before you leave, you'll have data access as soon as you land.

Of course, you don't have to use both SIM cards at the same time, or even at all, and when you buy the phone from your carrier, it usually comes with a standard SIM already installed and ready to connect to your account.

If you buy a phone that doesn't come with a SIM card, however -- from select retailers or websites like Swappa, for example -- you need to visit your carrier or order a SIM card and wait for it to arrive in the mail, then get it activated. With an eSIM, you don't have to wait -- you can activate the phone in minutes.

Angela Lang/CNET

Will all phones start using eSIM cards?

Odds are, yes. It makes sense for phone-makers and carriers to ditch the plastic SIM cards, both from an added expense standpoint, as well as environmental one, when you consider the large plastic card that a standard SIM card comes attached to.

Does my carrier have to support eSIM technology for me to use it?

Yes. Apple has a list of all the wireless carriers throughout the world that support the iPhone's eSIM feature.

As for Google's Pixel, the process is a little more confusing. Google Fi, which is Google's prepaid wireless carrier, can use an eSIM, and some US carriers such as AT&T and Sprint support eSIMs on the Pixel 4, but not on older Pixel phones.

When I attempt to add an eSIM mobile plan to my Pixel 4 XL ( ) now, I'm given the option of AT&T, Sprint and Google Fi.

It seems to be hit or miss when it comes to which carrier supports which device, and it gets even more confusing when you start looking into dual-SIM support.

If you have a Pixel phone and you want to use its eSIM functionality, it's a good idea to contact your wireless carrier and ask about eSIM support for your particular model.

Jason Cipriani/CNET

Phones aren't the only gadgets that use eSIMs

eSIM technology isn't only found in phones. The Apple Watch and Samsung's Galaxy Watch that has cellular connectivity use eSIMs to connect to your carrier. Some of Apple's iPad models with cellular also have an eSIM. We're also starting to see some laptops, such as the Surface Pro, that have eSIM capabilities.

How do I set up an eSIM?

The process varies based on your carrier. AT&T users will need to get an eSIM activation card from the carrier. It will have a QR code on it that you can use to program the eSIM with your information. Some carriers, like Verizon Wireless, use their mobile app to program the eSIM.

Again, it's a good idea to check with your carrier about its eSIM activation process.

Setting up an eSIM on your iPhone only takes a few seconds. If you want to see what life is like with an eSIM-only phone, the foldable Motorola Razr with its innovative hinge is the way to do it -- for a mere $1,499. The Galaxy Z Flip is available right now, has the same shape as the Razr, but is priced at only $1,380.

Juan Garzon/CNET

Originally published last year. Updated with information on new phones.