What is difference between a 220 volt outlet and a 240 volt outlet?

20 Volts - (the original answer). Nice math but not enough information.

My answer - 220, 230 and 240 volts are really all the same, just as 110, 115 and 120 are also. 240 volts single phase is what comes into most houses, into your power panel, using two "hot" wires and a neutral. Simply stated, each hot wire powers one side of the panel and delivers 120 "nominal" volts to each side and breakers are installed to provide 15 amp or 20 amp etc circuits to most household outlets and lights. When a breaker covers both sides of the panel, it can supply 240 volts at 30 amp or 40 amp or 50 amp etc. for things like dryers, ovens, water heaters, etc.

Each "nominal" 120 volt line can actually vary from about 112 volts to 128 volts, and you can't complain to the power company. For whatever reasons, devices and appliances will call for a 110 or 115 or 120 volt power source, but must be prepared to run at anything between 112 and 128.

Devices that run at 240 volts use both of the 120 volt lines to provide the increase in wattage required by the device (wattage = volts times amps).

208 Volts 3 phase (usually commercial) is different than 240 single phase (usually residential), but that's for another question.