What is the difference between a travel adaptor and a voltage converter?
An adaptor changes the shape of the plug so it fits the socket. A converter or transformer changes the electricity itself. They solve different problems, and using an adaptor when you needed a converter is how appliances get destroyed.
Two things vary between countries.
Plug shape — there are around 15 socket types in use worldwide, with different pin shapes, sizes and arrangements.
Voltage and frequency — most of the world runs on roughly 220–240V, while North America, Japan and parts of South America run on 100–127V. Frequency is 50Hz or 60Hz depending on the country.
An adaptor deals only with the first. Plug a 120V-only appliance into 230V through a simple adaptor and you will feed it roughly twice the voltage it was built for.
The good news is that most modern electronics do not care. Look at the small print on the charger or power supply: if it says "INPUT: 100–240V, 50/60Hz", it is dual voltage and an adaptor alone is all you need. Phone chargers, laptop power supplies, camera chargers and most tablets are almost universally dual voltage.
What is usually not dual voltage is anything that generates heat or uses a motor: hairdryers, straighteners, travel kettles, electric shavers of older design. These are the appliances that need a converter, and heat appliances draw enough power that many travel converters cannot handle them anyway.
A practical shortcut: buy dual-voltage versions of travel appliances rather than carrying a converter.
Some adaptors do not carry an earth connection, which matters for appliances with a metal casing.
USB-only adaptors are sufficient if everything you carry charges by USB.