The average microwave oven produces radiation at a wavelength of about 12 cm, which is in the range of microwaves. The frequency of this radiation is about 2.45 GHz. When this microwaves hit water molecules inside food, they cause the molecules to rotate rapidly. This friction creates heat, and it is this heat that cooks the food.
The amount of energy absorbed by the food depends on many factors, including the type of food, its moisture content, how much it has been cooked already, and the wattage of the microwave oven. In general, though, most microwaves will cook food at an internal temperature of between 70 and 80 degrees Celsius.