It's just an oven optimised for airflow.

Even fan-forced, a normal oven recirculates the same air around, which carries moisture evaporated from whatever you're cooking.

Air Fryer is just a much smaller oven, with a similar sized element and a bigger fan, that also has an exhaust, so moisture-laden air is expelled from the unit and the air hitting the food is always hot and dry.

But all-in-all it's just a type of oven and you can absolutely use one as a regular little oven or if you crack the door a little and turn the heat up a bit, you can use regular fan-forced oven as an 'air fryer'