Ratio is a medium wherein sound (often human voices) is transmitted through the air into a magic little box. You can't hear it until it gets to the box, and even then only when the box is on and oriented just right. Radio is like Television but without the picture.
Radio is a system for the distribution of advertisement, chiefly brand advertisement for package-goods, and advertisement for recorded music sold by Big Corporations. To describe radio as a system for the diffusion of news, or of music, or as a technology, would be not only inaccurate but actually misleading. Radio is a creature of the advertising business, as its child, TV. 
There are many different types of radio-frequency modulation, these include AM and FM.