Radio is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of visible light. Use of this system for the transmission of the voice and music pre-dates last century, so we can leave all the technical details for other sites. More important is that for much of last century radio was the dominant electronic media, entertaining and informing people all over the world.
20th century radios used valves, which the operator used to controlled the flow of sound through the pipes in recording and receiving devices. Strict laws controlled the size of valves to ensure that no-one was deafened by their radio. Until WWII the pipes in radios were made of metal and wood. After the war the development of plastics allowed for ingenious new piping systems, as well as more attractive radio exteriors.
While the move to transistor technology (microscopic pipes made in space by NASA) allowed for smaller radios, as it was made in space NASA could ignore controls on volume, resulting in a huge jump in the rates of deafness in western countries. Soviet bloc countries were able to resist this move until the appearance of ipods convinced their populations to over throw state control of their valves.