Radio is the transmission of signals, by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space. Early speculation that this required a medium of transport, called luminiferous aether, were found to be false. Information is carried by systematically changing (modulating) some property of the radiated waves, such as their amplitude or their frequency. When radio waves pass an electrical conductor, the oscillating fields induce an alternating current in the conductor. This can be detected and transformed into sound or other signals that carry information.

James Clerk Maxwell, a Scottish scientist, developed the theoretical basis for explaining electromagnetism. He predicted that electric and magnetic fields can couple together to form electromagnetic waves. Heinrich Hertz, a German scientist, is credited with being the first to produce and detect such waves at radio frequencies, in 1888, using a sparkgap transmitter in the Ultra High Frequency range.

In 1893, Nikola Tesla, in America, first demonstrated the feasibility of wireless communications. Guglielmo Marconi, an Italian inventor, was one of the first to develop commercial workable radio communication. It is supposed that he sent and received his first radio signal in Italy in 1895.
Etymology

Originally, radio or radioteleography was called 'wireless telegraphy', which was shortened to 'wireless'. The prefix radio- in the sense of wireless transmission was first recorded in the word radioconductor, coined by the French physicist Edouard Branly in 1897 and based on the verb to radiate (in Latin "radius" means "spoke of a wheel, beam of light, ray"). 'Radio' as a noun is said to have been coined by advertising expert Waldo Warren (White 1944). The word appears in a 1907 article by Lee de Forest, was adopted by the United States Navy in 1912 and became common by the time of the first commercial broadcasts in the United States in the 1920s. (The noun 'broadcasting' itself came from an agricultural term, meaning 'scattering seeds'.) The term was then adopted by other languages in Europe and Asia, although British Commonwealth countries retained the term 'wireless' until the mid-20th century. In Japanese, the term 'wireless' is the basis for the term 'radio wave' although the term for the device that listens to radio waves is literally 'device for receiving sounds'.

In recent years the term 'wireless' has gained renewed popularity through the rapid growth of short range networking, e.g., WLAN ('Wireless Local Area Network'), WiFi and Bluetooth as well as mobile telephony, e.g., GSM and UMTS. Today, the term 'radio' often refers to the actual transceiver device or chip, whereas 'wireless' refers to the system and/or method used for radio communication. Hence one talks about radio transceivers and Radio Frequency Identification (RFID), but about wireless devices and wireless sensor networks.
Although invention was long attributed to Guglielmo Marconi, the identity of the original inventor of radio, at the time called wireless telegraphy, is contentious[1]. Development from a laboratory demonstration to commercial utility spanned several decades and required the efforts of many practitioners. The controversy over who invented the radio, with the benefit of hindsight, can be broken down as follows:
In 1887, David E. Hughes transmitted signals by radio using a clockwork-keyed Spark Transmitter, achieving a range of approximately 500 metres.
In 1888, Heinrich Hertz produced and measured the Ultra High Frequency range (via a sparkgap transmitter).
In 1891, Nikola Tesla began wireless research. He developed means to reliably produce radio frequencies, publicly demonstrated the principles of radio, and transmitted long-distance signals.
Between 1893 and 1894, Roberto Landell de Moura, a Brazilian priest and scientist, conducted experiments. He did not publicise his achievement until 1900 but later obtained Brazilian patent.
In 1894 in Kolkata (Calcutta), Sir Jagdish Chandra Bose (J. C. Bose) invented the mercury coherer, together with the telephone receiver.
Alexander Stepanovich Popov, in 1894, built his first radio receiver, which contained a coherer but actually coherer was first invented by Edouard Branly. Popov demonstrated the coherer, further refined as a lightning detector, to the Russian Physical and Chemical Society on May 7, 1895.
In 1894, Guglielmo Marconi read about Hertz's and Tesla's work on wireless telegraphy, and began his own experiments.
In December of 1901 Guglielmo Marconi used J.C. Bose's inventions to receive the radio signal in his first transatlantic radio communication over a distance of 2,000 miles (3,200 km) from Poldhu, UK, to St. Johns, Newfoundland. Marconi was celebrated worldwide for this achievement. Soon after the patent was given to Marconi. He even received the Nobel Prize.
In early 1900s Reginald Fessenden and Lee de Forest invented amplitude-modulated (AM) radio) allowing an audio signal to be sent over the air.
In 1935 Edwin H. Armstrong invented frequency-modulated (FM) radio, so that an audio signal can avoid "static," that is, interference from electrical equipment and atmospherics.
In 1943, the U.S. Supreme Court acknowledged that Marconi's work wasn't original, and the patent ownership is given back to Nikola Tesla. However, Tesla died shortly before the decision was announced.
Uses of radio

Early uses were maritime, for sending telegraphic messages using Morse code between ships and land. The earliest users included the Japanese Navy scouting the Russian fleet during the Battle of Tsushima in 1905. One of the most memorable uses of marine telegraphy was during the sinking of the RMS Titanic in 1912, including communications between operators on the sinking ship and nearby vessels, and communications to shore stations listing the survivors. The first radio couldn't transmit sound or speech and was called the "wireless telegraph"

Radio was used to pass on orders and communications between armies and navies on both sides in World War I; Germany used radio communications for diplomatic messages once its submarine cables were cut by the British. The United States passed on President Woodrow Wilson's Fourteen Points to Germany via radio during the war. Broadcasting began from San Jose in 1909[8], and became feasible in the 1920s, with the widespread introduction of radio receivers, particularly in Europe and the United States. Besides broadcasting, point-to-point broadcasting, including telephone messages and relays of radio programs, became widespread in the 1920s and 1930s. Another use of radio in the pre-war years was the development of detecting and locating aircraft and ships by the use of radar (RAdio Detection And Ranging).

Today, radio takes many forms, including wireless networks and mobile communications of all types, as well as radio broadcasting. Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows, and many other forms of entertainment. Radio was unique among methods of dramatic presentation in that it used only sound. For more, see radio programming.