A radio is a device that transmits or receives electromagnetic waves to send or receive sound (usually audio) signals wirelessly. A radio receiver tunes into specific frequencies, demodulates the signal, amplifies it, and converts it into sound via speaker/headphone; a radio transmitter does the reverse (takes sound or data, modulates onto a carrier wave, and broadcasts).
A transmitter generates a carrier wave at a particular frequency.
The audio or data you want to send is used to modify (“modulate”) that carrier (for example by changing its amplitude or frequency).
The modulated signal is broadcast via an antenna.
A receiver picks up many such signals, but you tune it so that only the desired frequency resonates.
The receiver demodulates (extracts the audio/data), amplifies it, and converts it to sound through a speaker.
AM stands for Amplitude Modulation. In AM, the amplitude (strength) of the carrier wave is varied in proportion to the audio signal. The frequency stays (mostly) constant. Commonly used for talk radio, news, etc. Pros: long range, especially useful at night. Cons: more susceptible to noise/interference, lower fidelity.
