Microwaves and Microwave Oven
A microwave oven, or a microwave, is a kitchen appliance that cooks or heats food by dielectric heating. This is accomplished by using microwave radiation to heat water and other polarized molecules within the food. This excitation is fairly uniform, leading to food being adequately heated throughout (except in thick objects), a feature not seen in any other heating technique.
Microwaving food raises several safety issues, largely connected with leakage of microwave radiation outside the oven, as well as reducing risks, such as that of fire from high temperature heat sources. There has been some concern that microwaves might damage food (microwave radiation has sounded alarming to some), but the dominant view is that microwaved food is as safe to eat as other food.
The microwaves emitted by the source in a microwave oven are confined in the oven by the material out of which microwave oven is constructed. Tests have shown confinement of the microwaves in commercially available ovens to be so nearly universal as to make routine testing unnecessary. According to the United States Food and Drug Administration's Center for Devices and Radiological Health, a US Federal standard limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts of microwave radiation per square centimeter at approximately 2 inches from the surface of the oven. This is far below the exposure level currently considered to be harmful to human health.
The radiation produced by a microwave oven is non-ionizing. It therefore does not have the cancer risks associated with ionizing radiation such as X-rays, ultraviolet light, and high-energy particles. Long-term rodent studies to assess cancer risk have so far failed to identify any carcinogenicity from 2.45 GHz microwave radiation even with chronic (i.e., large fraction of life span) exposure levels, far larger than humans are likely to encounter from any leaking ovens. However, with the oven door open, the radiation may cause damage by heating; as with any cooking device. Nearly every microwave sold has a protective interlock so that it cannot be run when the door is open or improperly latched.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)
For more information (background, pictures, experiments and references): Microwave Oven