Turbojets are a type of air breathing jet engine. They are one of the simpler types of jet engines. Unlike some other jet engines such as turbofans, they do not have a fan at the front to suck in air. Instead they rely on air going into the inlet as they travel forwards through the air. This means that they are not effective at low speeds, but can be very effective at higher speeds, such as in supersonic aircraft.
Since there is no fan on the front of a turbojet to suck in air, the power from the jet's turbine goes solely to the compressor, so less energy is taken away from the jet stream coming out the back.