Signalling
Without actually going into particle or wave theory, all signals are part of the electromagnetic spectrum. All energy in the universe travels in waves and radiates outwards from a source. All radiation is said to have a wavelength (frequency) and amplitude. Everything from gamma rays all the way down through the visible light spectrum to radio waves are all electromagnetic energy and differ only in frequency. Frequency is measured in meters, and amplitude is measured in electron volts.
Various devices are used to generate electromagnetic energy and broadcast it. Signals can be transmitted over copper, the air or even through glass or clear plastic. When we use electromagnetic energy to transmit information, it's called signalling.
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