A transformer (also known as a power converter or voltage converter) is a device that changes the electrical output (voltage) of a power source. When an electronic device was built for a specific voltage (not a variable voltage), but you need to use it in a country where that voltage is not available, the device can only be used if the power is converted up or down to the correct level. The need for a transformer often arises for people who are travelling and bringing electronics with them.
Step Up and Step Down Transformers
Step Up voltage transformers are used to "step up" the electricity in the countries that use 100-127V so you can use 220-240V devices. For example, if you are travelling to the USA where the power outlets are of 110V, but coming from Australia and have a device with you that is only suitable for 230V (i.e. a device that doesn't have variable voltage), you may use a step-up transformer to convert the 110V power outlets in the USA to 230V so you can safely use your device.
Step Down voltage transformers are used to "step down" the electricity in the countries that use 220-240V so you can use 110V devices. For example, if you are travelling to Australia where the power outlets are of 230V, but you are coming from the USA and have a device with you that is only suitable for 110V (i.e. a device that doesn't have variable voltage), you may use a step-down transformer to convert the 230V power from outlets in Australia to 110V so you can safely use your device.
Many transformers available today are 2-way transformers, meaning they are capable of acting as both step-up and step-down transformers.
Do I need a transformer?
Some appliances are compatible with multiple voltages. To find out if your appliance is one of them, check the appliance label and/or instruction booklet. Some examples of appliance labels are below: