The main difference between 120V and 240V is the amount of electrical power that flows through the wires. Outlets with a 240V rating supply twice as much electrical power as those rated 120V.
The standard rating for appliances in the United States is 120V, so most electrical outlets are 120V; however, some bigger appliances requiring more electrical power to operate use 220V or 240V. Most other countries on continents other than North America run on 220 or 240V power. This means that a voltage converter or step down transformer is needed to use a 120V rated appliance in a 240V outlet.