Take an NFC business card apart and there is not much in it. A coil of wire runs around the inside of the card, and a tiny chip sits at the end of it. The chip holds one web address. Everything else you see when somebody taps it, your photo, your title, the Save contact button, lives on a web page.
That design explains most of what people ask about these cards, from whether they are private to why they never need charging.
The chip has no battery
An NFC card is passive. When a phone comes close, its reader produces a field at 13.56 MHz, and the coil in the card picks up enough energy from that field to wake the chip and answer. Take the phone away and the card goes back to doing nothing.
This is why the range is short. The card has to sit right against the phone's antenna to get enough power, which is why a thick case or a metal ring holder can stop a tap from reading. And because it does nothing until a phone powers it, the card does not wear out sitting in your wallet.
How much it can hold
Very little, and that is fine. NXP's NTAG 213, 215 and 216 are a good example. They are NFC Forum Type 2 tags, and NXP lists their free user memory at 144, 504 and 888 bytes. A short paragraph of plain text would fill the smallest one.
A business card stores its link as an NDEF message with a URI record, the format both Android and iPhone look for. The record saves room by turning the common start of an address into a single code byte. Android's documentation shows one byte standing in for https://www. at the front of a link. A profile address like a domain plus your name comes to a few dozen bytes, well inside even the 144 byte chip.
So when a seller talks about a big memory chip, ask what it is for. If the card only opens your profile, the extra memory sits empty.
Why a card should store a link, not your details
You could write your name and number straight onto the chip as a contact record. Then every change means rewriting the card, and anyone who reads it gets a copy you can never update.
Storing a link moves your details onto a page you control. Change your title or your number and every card you ever handed out opens the new version. Lose the card and the finder learns nothing your profile was not already showing in public. We go through that case in what happens if you lose the card.
How long it lasts, and who can change it
NXP rates the NTAG 21x chips for 10 years of data retention and 100,000 write cycles. Reading does not count against the write limit, so a card that gets tapped every day for years is not using anything up. In practice the plastic and the antenna take more abuse than the memory does.
Rewriting is the part worth checking. A blank tag can be rewritten by any phone with the right app. NXP's chips support locking and 32 bit password protection, with an optional limit on failed attempts, so a card can be set up so that a stranger cannot change where it points. They also carry an originality signature, which lets software check that the chip is a genuine NXP part.
What the other person's phone does
On iPhone, the phone looks for a URI record with a web address or another supported link. On models that read tags in the background, the system shows a notification for the link, and tapping it opens the page. Apple lists the times when that background reading is off: while the camera or Apple Pay Wallet is in use, in airplane mode, during an app's own NFC session, and when the phone has never been unlocked.
On Android, the phone looks for tags whenever the screen is unlocked, unless NFC is switched off in Settings. Google's documentation says that from Android 16, a tag that holds a web link opens it like any other link. From Android 17, the phone first shows an open link notification, and the person taps it to continue. That is one extra tap, and it means a page never opens unless the person chooses it.
Either way, nothing is installed and nothing is shared back. If you are still unsure which phones read these cards at all, we covered that in does a tap card work on every phone.