James Clerk Maxwell was what many may think of as a theoretical scientist. He developed a set of equations, called the Maxwell equations. The Maxwell Equations mathematically describe electricity and magnetism, and show that electric and magnetic fields can act together, forming electromagnetic waves:
Electromagnetic waves include radio waves, x-rays, gamma rays -- and light!
Heinrich Hertz was more of an experimental scientist. He decided to demonstrate Maxwell's equations by generating and detecting invisible electromagnetic waves. Hertz built equipment that made the first radio waves in 1887.
Guglielmo Marconi was one of the first electrical engineers. He read about Hertz' experiments, and a saw the possibilities of using radio waves for COMMUNICATION. Marconi built his own version of a radio transmitter and receiver, and used it to send messages - at first by Morse code. Marconi was later the first person to send messages by radio waves to ships at sea -- and, later yet, across the Atlantic Ocean from Europe to America.