A step-down converter is used to step electricity down from 220 volts to 110 volts. Step-down converters are used when traveling outside of the United States, where many countries' electrical systems use 220, 230 or 240 volts.
Without a step-down converter, appliances manufactured for a 110-volt outlet will suffer a shock failure when plugged into a 220-volt outlet. While adapters are available, the adapter is only for the outlet pattern; it doesn't change the voltage of the outlet.
Step-up converters are also available for items purchased outside of the United States, allowing the buyer to use a 220-volt appliance with a 110-volt outlet when he or she returns home. Some models may be used for both stepping up and stepping down the electricity, making it easy for a traveler to use a hair dryer or electric razor in any country.