The speeds within a sound propagating medium like air could be compared to the speed of sound. The speed of sound could be imagined as something four times faster than a race car. Speed lower than the speed of sound is called subsonic speed. An aircraft that travels lower than the speed of sound in air is called subsonic flight.
Airplanes that travel faster than the speed of sound are powered by jet engines. They are referred to as ‘supersonic’ planes. Supersonic speed is the rate of travel of an object that exceeds the speed of sound. Most of the modern fighter aircrafts are supersonic aircrafts. In the olden days, there were two supersonic passenger flights namely Concorde and Tupolev Tu-144. These flights were able to take passengers from London to New York in less than 3.5 hours. These are no more in service today. Bullets fired from a gun travels at supersonic speeds.
The way air flows around the aircraft distinguishes these categories. At subsonic speed, which is below the speed of sound, shock waves are absent. At supersonic speeds, shock waves form on the aircraft as it flies through the air. Very highly supersonic waves are called hypersonic waves. Rocket propelled aircrafts like space shuttle travel at hypersonic speeds on return from space. At hypersonic speeds, the shock waves form very close to the aircraft, and engineers are developing ways to harness the power of these waves and use them for aiding fuel combustion inside the engine of the aircraft.