Wikijunior:How Things Work/Microwave oven
A Microwave oven is designed to heat most foods in a quick and convenient way. The name Microwave is derived from the energy used to cook the food, microwaves, which pass through the cells and molecules of the food, the frequency of the waves causes the water molecules to vibrate, this movement generates heat. These microwaves are produced by a device called a magnetron within the microwave oven.
Who invented it? 
Dr. Percy Spencer, a self-taught engineer with the Raytheon Corporation observed the magnetron tube's radiative effects when a candy bar placed in his pocket melted while testing a new vacuum tube called a magnetron. He then tried this with oiled corn (which popped) and an egg (which exploded,) and observed rapid heat development and eventual cooking of these foods.
How does it get power? 
The oven is attached to a power socket by a plug. This feeds electricity into the oven, powering it up!
How does it work? 
An internal device called a magnetron is inside the oven. When you press 'Start', the magnetron starts emitting microwaves, which are like radio waves but at a higher frequency (the length between the waves is much shorter, so they pack more zappo and therefore more energy.) These waves bounce up and down and left and right around the oven. When they come in contact with the food, the energy from the microwaves causes molecules in the food to start moving around. This is basically what heat is -- molecules getting excited and moving around rapidly -- so this means the temperature rises. That then cooks the food! Microwaves work especially well to heat water molecules, but they also heat fats and sugars in your food. This is why foods with fats and sugars in them heat up much faster and much more than foods without them.
How dangerous is it? 
A microwave oven is not very dangerous, but it does have electric components inside that could be dangerous if you took the microwave apart. A metal screen on the door prevents microwaves from escaping the oven while it is operating. Once the oven stops, the microwaves disappear immediately so are not dangerous when you open the door. However, if there is a hole in the door or the microwave malfunctions, microwaves could escape the oven and burn or cook you if you're standing right in front of it. It has also been claimed that leaking microwaves could cause birth defects in pregnant women. You should not put metal objects in a microwave, such as a fork, because it could cause arcing, which will damage the oven. Another danger of microwaves is that, if heated for too long, water could become superheated -- heated above its boiling point, even though it doesn't appear to be boiling. It could abruptly boil when you move it, scalding you.
What does it do? 
Microwaves seem to cook food from the inside out.
How does it vary? 
Ovens vary in wattage (how much power they cook with). A medium power microwave might be 1,000 watts, or a kilowatt. A larger one might be 1.5 kilowatts.
How has it changed the world? 
Microwave ovens have made cooking and reheating food much much much MUCH MUCH easier.
What idea(s) and/or inventions had to be developed before it could be created? 
As mentioned above, the magnetron had to be invented before microwaves could be used for cooking.