The wavelength of a microwave can range from 1 millimeter (.001 meter) to 1 meter. This range is actually within the broader term radio wave, but on some scales, they are separate. Accordingly, the frequency range of microwaves is 300 MHz to 300 GHz (300,000,000 to 300,000,000,000 Hz).
Microwaves are specifically used in areas such as communication, power, and radar. Before fiber-optic cables were adopted into the phone system, microwaves were used for the same purpose. Radar, or the ability to map objects by bouncing waves off them, lies predominantly within the microwave region, but perhaps the most famous of microwave uses is the microwave oven. Microwave ovens bombard non-radioactive radiation into food, installing energy into, and therefore heating, it.
The main component of the microwave oven is the magnetron. This is a cross-section of such a device which generates an electric field to produce microwaves. Magnetrons are generally for heating certain substances with microwaves, such as water, sugar, and fats in food, and sometimes, humans. Microwaves, however, are not specifically heat radiation.
Microwaves, just like all waves used for communication are divided into bands to organize their use. WIth microwaves, these bands are denoted by letter, (e.g. the L Band, ranging from 1 to 2 GHz) and each band has specific uses.
Yet another use of microwaves is in astronomy. There are many sources of microwaves in the heavens, but the most notable is the Cosmic Microwave Background Radiation. This radiation was emitted 379,000 years of the Big Bang, when the Universe became transparent due to the cessation of photon-plasma reactions. Plasma is ionized gas, and is therefore made of ionized atoms. When the temperature of the Universe had fallen enough to support complete atoms with electrons, the atoms became neutral, and the reactions stopped. The image of the Universe released at that moment was carried by gamma rays, but over time red shifting transferred these into microwaves, as they are today. More and less intense patches on the radiation indicate the density of different parts of the Universe. The differences were minute at the time, but gravity gradually increased the differences, and all of the structure in the large scale Universe was defined.
An image adjusted from the original Cosmic Microwave Background to show temperature differences in the early Universe.
Microwaves have a variety of uses, from communication to heating to discovering the origins of the Universe.
Sources: http://en.wikipedia.org/wiki/Microwave, http://www.gallawa.com/microtech/mwave.html, http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation, etc.
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