1. The plain mechanical key
Metal, no buttons, no plastic head with electronics in it. It opens the door and starts the engine and does nothing else. Found on older vehicles, on many trailers, and on some light commercial vehicles.
This is the simplest key to replace — it is a cutting job with no programming at all.
2. The transponder key
Looks like a normal key with a slightly chunky plastic head. Inside that head is a chip. When the key is in the ignition, the car challenges the chip, and if the answer is wrong the engine will not start no matter how perfectly the key is cut.
This is the one that surprises people: a locksmith can cut a key that turns the barrel beautifully and the car still will not start. The cut and the chip are two separate pieces of work, and both have to happen.
3. The remote key or flip key
A transponder key with buttons — lock, unlock, sometimes a boot release. Often a flip key where the blade folds into the body. The remote function is separate from the immobiliser chip, which is why one can fail while the other still works.
A remote that has stopped working is usually a battery, sometimes a lost pairing, and only occasionally a genuinely failed unit. Those are three different jobs.
4. The proximity or smart key
No blade in normal use, and it never leaves your pocket. The car senses it nearby, unlocks on a door handle touch, and starts on a button. Most of these hide a small emergency blade inside the fob for the day the battery dies.
A proximity key is in constant conversation with the vehicle rather than sending a one-way signal, so replacing one is a pairing job rather than a simple programming job. Routine, but not the same work.
Why this matters on the phone
"I've lost my car key" describes four different jobs with four different equipment requirements. "I've lost my proximity key for a 2019 hatchback" gets you a real answer in the first call, and gets the right gear onto the van before it leaves.
Have the make, model and year ready as well. Key systems change between model generations and occasionally within them, which is why no honest answer about your specific vehicle can be given from a web page.
The one that applies to all four
Whatever kind you have, two working keys is the sensible minimum for any car you rely on. The difference between duplicating a key and originating one from nothing is the whole reason spares are worth doing early.