Microwaves are a form of electromagnetic radiation. They can pass through most materials except metals, ceramics and water. They are used in point-to-point telecommunications systems to convey analog and digital voice, data and video, in industrial applications such as supervisory control and data acquisition (SCADA) for remote machinery, switches and valves, and for air traffic control and microwave ovens. They are also used in radar (RAdio Detection and Ranging) for weather tracking, airport landings and the speed-of-light measurement of starships and rockets.
They are produced by special vacuum tubes referred to as klystrons, magnetrons and Gunn diodes. These devices use different principles than low-frequency vacuum tubes, working on the basis of clumps of electrons flying ballistically in a vacuum under the influence of controlling electric and magnetic fields.
In a microwave oven, a filament emits microwaves as it heats up. Ring magnets above and below the filament bend the electrons in a circle. This creates a microwave beam that radiates into the cooking compartment. The reflected microwaves heat the food, which is why microwave directions usually tell you to move or stir the food or wait a few seconds before eating.
Microwaves ricochet around inside the cavity, and just like waves in a bathtub, they have peaks (lots of microwaves) and troughs (little microwaves). Your microwave rotates the dish periodically so that the food is exposed to the maximum number of peaks. The rotating action also helps even out the cooking, which is why older microwaves without a spinning wheel had to be moved manually or flipped over partway through the heating cycle.