Short answers, pulled from the story.
Radio is the technology of communicating using radio waves, electromagnetic waves with frequencies between 3 hertz and 300 gigahertz. A transmitter connected to an antenna radiates the waves, and another antenna connected to a receiver picks them up, which is the fundamental principle of radio communication.
German physicist Heinrich Hertz first observed radio waves on the 11th of November 1886, using a primitive spark-gap transmitter while attempting to confirm Maxwell's theory of electromagnetism. James Clerk Maxwell had predicted the waves earlier, in his 1873 theory now called Maxwell's equations.
Guglielmo Marconi developed the first radio communication system in 1895, using a spark-gap transmitter to send Morse code over long distances. By December 1901 he had transmitted across the Atlantic Ocean, and in 1909 he shared the Nobel Prize in Physics with Karl Ferdinand Braun for contributions to wireless telegraphy.
The first commercial radio broadcast was transmitted on the 2nd of November 1920 by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign KDKA. It featured live coverage of the 1920 United States presidential election.
The word radio derives from the Latin radius, meaning spoke of a wheel, beam of light, or ray. It was first applied to communications in 1881 when Alexander Graham Bell adopted radiophone, meaning radiated sound, for his photophone optical transmission system.
The emission of radio waves is regulated by national laws and coordinated by the International Telecommunication Union, which allocates frequency bands and divides the spectrum into twelve named bands. Transmitters must be licensed and restricted to certain frequencies and power levels, with broadcasting stations identified by a unique call sign.
Beyond broadcasting, radio is used for radar, radio navigation systems such as GPS and VOR, remote control, remote sensing, two-way radios, cell phones, wireless networking, and satellite communication. Radar locates and tracks objects like aircraft, ships, spacecraft, and missiles by reflecting radio waves off them.