Hertz demonstrated that passing alternating current through a wire caused it to emit corresponding electromagnetic waves. When those waves are detected by a receiving antenna, they induce a matching current, as shown below.
Like a pebble dropped in a pond, if the transmitter has enough power to reach and be detected by the receiver, it can be used to communicate over a distance.
The signal attenuates (loses power) as it radiates out from the transmitting antenna. If the signal level is above the sensitivity threshold of the receiver when it arrives, it can be detected and decoded.
You have experienced attenuation with your cell phone or car radio. Your car radio may sound fine in the city, but fade when you drive out of town. Similarly, some parts of your city may not be close enough to a cellular radio tower to connect and talk.
A radio signal can also be drowned out by interfering radiation or noise. Low quality portable radios often have problems separating the signals from two stations that transmit at similar frequencies. Home appliances and power tools may also interfere with radio reception.
An obstruction can also attenuate or completely block a radio signal. Your car radio fades out when you drive through a tunnel, and WiFi signal strength is diminished by walls, trees, etc.
All electromagnetic waves (including radio waves) propagate at the same speed, the speed of light (just under 300 million meters per second in a vacuum). That being the case, we can convert a radio wave's frequency into its wavelength. For example, if the frequency is 1 Ghz, the wavelength must be .3 meters.
We use the term radio to describe communication using electromagnetic waves with frequencies in the 3 Khz to 300 Ghz range. Signals above 100 Mhz are often called microwaves since they have short wavelengths. Infrared radiation, visible light, ultraviolet, X-rays, and gamma rays have frequencies even higher than radio waves and; therefor, shorter wavelengths.
We take radio communication for granted today, but it seemed like magic at first. Electromagnetic waves can propagate through a vacuum. They do not propagate by deforming a medium like air or water. Rather, an oscillating electric field creates an oscillating magnetic field, which in turn creates an oscillating electric field, and so on. This was not obvious to physicists. They noted that light traveled to the earth from stars, and continued looking for a light carrying substance, a "luminiferous aether," in outer space well into the 20th century.
When Albert Einstein was asked to explain radio, he said:
You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat."
Today we use radio for a wide range of communication applications like cellular telephones, car radios, transmitting photos from Mars, satellite television, and local and personal area networks.