From the phisical point of view...

Microwaves are electromagnetic waves with short wavelength starting from 1mm to 1m. It is a type of radiation that is emitted as mutually infiltrating electric and magnetic oscilations.
Microwaves range of working for short waves are restricted by infrared waves, longer waves point out generation and transmission technics that are associated with using power tubes and special industry lines.

Microwaves range of working for short waves are restricted by infrared waves, longer waves point out generation and transmission technics that are associated with using power tubes and special industry lines.

A bit of history

The age of microwaves started when radiowaves were discovered. In 1888 German physician Heinrich Hertz discovered that electric stroke of big spark caused stroke of small one in the tube that was placed in small distance. This experience led to discover radio and microwaves, that are small part of radio radiation. The first device that could generate microwave radiation was constructed in England during the second world war as a part of radar devices for finding moves of German troops. In those times, it was noticed that this kind of radiation has big influence on preheating many substances i.e. water.

Microwaves sources and the ways of emission

High frequency current that flows in antennas of transmission devices is the most common microwave radiation source. Microwaves are created in a similar way as radio waves are but with higher frequency. Sending electromagnetic waves in the technic of microwave requires solving problems with coded informations in modulated amplitude of current flowing form. Propagation of microwaves is a bit restricted regarding area of visibility due to the fact that waves are not liable for reflections and interferences from ionosphere.

Specification

Microwaves are liable to reflection from the high thinckess objects, they are also liable to emission and silencing in athmosphere. Silencing is mostly caused by raining or snowing. Their intensity depends on the wave length. Characteristic effect of microwaves is echo that is used in radars. In radiocomunication echo is not welcome because it may disturb quality of signal. Microwaves are also used in phone and TV transmissions.

The Microwave radiation are liable to the same rules as in ultraviolet and infrared radiation. The difference between other electromagnetic waves is that the microwave radiation causes rotation of molecules in alternatic electric field without touching any chemical compounds. The energy that is transmitted by microwave radiation is much smaller than energy of chemical compound dissociation. Dielectric polarization is responsible for microwave heating. It is the result of chemical compounds moleculs polaryzation by electric field.

Microwaves has an influence on living beings. In case of human beings it may cause increase of body temperature, feeling of being sleepy, tired, nervous, headache, problems with memory, changes in metabolizm. It is told that safety zone is in the area where medium thickess of microwave power source is smaller than 0,1 W/m². When the zone thickness is higher than 100 W/m² it is severly danger for people and they should avoid places like that.

Where do we use it?

Microwave radiation is used mostly in radars. Radar is an acronym for "radio detection and ranging". Radar was developed to detect objects and determine their range (or position) by transmitting short bursts of microwaves. The strength and origin of "echoes" received from objects that were hit by the microwaves is then recorded.

For the fact that microwaves can penetrate haze, light rain and snow, clouds and smoke, they are good enough for viewing the Earth from space. In 1964 first echo from Venus was registered. Then, within short period of time echoes from other planets like Mars or Mercury were also registrated. Radar technics that use microwaves are also used in plains transporders- these are special radioloaction devices.

Microwaves are also used as meteorological and police radars. And, of course, microwave oven is indispensable AGD device for almost everyone in the kitchen. Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, therefore rotate as they try to align themselves with the alternating electric field induced by the microwaves. This molecular movement creates heat as the rotating molecules hit other molecules and put them into motion. Thanks to this process we may cook food much faster than in traditional oven. 

Microwaves radiation has variety of use in telecomunication, meteorology, chemistry and in our houses. Military and telecomunication use of those waves exclude using of several microwave frequencies. Usually it is 2,45 GHz. However, it is allowed to use 0,915 GHz to 18GHz  frequencies in machines that are working in the industry. Microwaves may be used in radiolocation, radiocomunication(radio, TV, phones) satelite transmisions, radioastronomy and medicine.

Nowadays, scientists are studing the way of transmitting energy with help of microwaves.