On the plug side, the distinction that costs people money and occasionally equipment.
An adapter changes the shape of the plug. That is all it does. This is what you need in almost every case, because nearly all modern chargers, laptops and phone bricks accept a wide input range: check the small print on yours and it will say something covering both common mains voltages. If it does, an adapter is sufficient anywhere.
A converter changes the voltage. Bulky, heavy, and only needed for things with a heating element or a motor that are built for one voltage: a travel kettle, a hairdryer, some shavers. Using one on a modern charger is unnecessary and some converters are actively bad for electronics.
So the airport purchase to avoid is the heavy universal converter. Buy a small adapter, or a set if you travel widely.
Two practical notes:
Get an adapter that grips. The cheap universal ones are loose and a heavy charger falls out of the socket, which is how people lose them in hotel rooms.
Check whether it carries earth. Some laptop chargers have a three-pin plug for a reason and a two-pin adapter defeats it. Not a safety catastrophe for a double-insulated charger, and worth knowing which you have.