Radio covers a variety of technologies by which information is imposed on electromagnetic radiation traveling without wires, through air or space. Modulation describes the set of processes of imposing the information on the unmodified carrier wave.
Radio frequency aspects
Modulation falls into two categories: those appropriate for carrying analog signals and those appropriate for digital signals. In understanding radio systems, you will have to understand how the information signal modulates the carrier wave, but also how the modulated signal is demodulated to extract the information that is being carried.