What is Radio waves?

Radio waves: The longest wavelengths with lowest frequency

Q: What it the electromagnetic spectrum?
A: The Electromagnetic spectrum or EM spectrum is the vast range of all different types of electromagnetic radiation. The EM waves have the same vibrations but at a different frequencies.
Here are the graphical representation:

Q: What is the historical discovery?
A:

In 1864, a Scottish theoretical physicist and mathematician James Clerk Maxwell (June 13,1831- November,1879) established the famous equations that bear his name and include the laws of electrostatics and magnetism.

His theory of electromagnetism  is the basis of current designs.

Maxwell's Law of Unification

In 1886, a German Physicist, Heinrich Rudolph Hertz demonstrated the existence of radio waves.

Here are Hertz Equation:

Frequency = (wave speed in m/s) / wavelength λ in m) or  f = c / λ

Time = 1 / Frequency or T = 1 / f

The unit  hertz (Hz) was once called cps = cycle per second