A plug adapter does one thing

It rearranges the pins. There is nothing inside but brass, so a US Type A plug becomes a UK Type G plug, or a European Type C becomes Australian Type I. It does not change voltage, current or frequency.

Because it is passive, quality matters less than people think for small loads — but check two numbers: the amp or watt rating (cheap adapters often stop at 6 A / 1380 W) and whether the earth pin actually connects. An adapter that passes live and neutral but ignores earth turns an earthed appliance into an unearthed one.

A converter changes the electricity

A voltage converter steps 220–240 V down to 110–120 V, or up the other way, so a single-voltage device sees something like the supply it was built for. There are two technologies:

  • Transformers are heavy, reliable, and suited to resistive loads such as hairdryers and travel kettles.
  • Solid-state converters are light and cheap, fine for shavers and electronics at low wattage, and prone to overheating at high wattage.

In both cases the converter must be rated above the device’s wattage — a 1600 W hairdryer needs a 2000 W converter, not a “travel” one rated 50 W.

Read the label, not the plug

The only reliable test is the small print on the device or its power supply:

  • INPUT: 100–240 V, 50/60 Hz — dual voltage. Adapter only, anywhere in the world. Nearly all phones, laptops, tablets, camera chargers and toothbrushes are built this way.
  • INPUT: 120 V 60 Hz — single voltage, North America. On a 230 V grid you need a converter.
  • INPUT: 230 V 50 Hz — single voltage, Europe. On a 120 V grid it will underperform or refuse to start; a step-up converter is needed.

What about frequency?

Adapters and most converters leave frequency alone. Electronics that rectify to DC do not care, so chargers, laptops and LED lamps are unaffected by 50 versus 60 Hz. Synchronous motors do care: a 50 Hz fan on 60 Hz runs about 20% faster, and mains-powered clocks keep the wrong time. Anything with a heating element is indifferent to frequency but very sensitive to voltage.

The short version

Adapter if the voltage matches and only the pins differ — or if the device says 100–240 V.
Converter if the device names a single voltage and the destination uses the other one.
Both when you are crossing voltage classes with a high-power appliance: a converter usually has its own plug shape, so you may still need an adapter on the wall side.