A ramjet is designed around its inlet. An object moving at high speed through air generates a high pressure region upstream. A ramjet uses this high pressure in front of the engine to force air through the tube, where it is heated by combusting some of it with fuel. It is then passed through a nozzle to accelerate it to supersonic speeds. This acceleration gives the ramjet forward thrust.
A ramjet is sometimes referred to as a 'flying stovepipe', a very simple device comprising an air intake, a combustor, and a nozzle.
Normally, the only moving parts are those within the turbopump, which pumps the fuel to the combustor in a liquid-fuel ramjet. Solid-fuel ramjets are even simpler.
By way of comparison, a turbojet uses a gas turbine-driven fan to compress the air further. This gives greater compression and efficiency and far more power at low speeds, where the ram effect is weak, but is also more complex, heavier and expensive, and the temperature limits of the turbine section limit the top speed and thrust at high speed.