A microwave oven is a kitchen appliance that heats food by dielectric heating. This is accomplished by using microwave radiation to heat polarized molecules within the food. This penetration leads to food being more evenly-heated throughout, than occurs in other air-based cooking techniques.
Microwaving is well known for causing problems if you put metal in it, but it also results in some interesting effects when used on different foods.
Here are some awesome GIFs of food during the microwaving process.