In all three of these types of jet engines, air from the front enters, is compressed, and is mixed with fuel. Then the fuel is burned and the resulting hot gas leaves out the back of the engine. Because the fuel heats up the air, it's hotter when it leaves than when it enters, so you get more energy from letting it expand than you spent on compressing it.

In a turbojet, a fan at the front pulls in and compresses the air, and an inverted fan called a turbine captures some energy from the departing air to keep the fan spinning. In a ramjet or scramjet, the engine draws in air by flying forward into it ("ramming" into the air), and compresses the air by having internal geometry that air has to increase in pressure to pass by.

The difference between a ramjet and a scramjet is that in a ramjet, the compressing geometry slows the compressed air to below the speed of sound before the added fuel starts burning, whereas in a scramjet the combustion happens in air that's moving faster than sound. "Scramjet" is short for "supersonic combustion ramjet."