Thrust is the force needed to overcome the resistance of air (drag) to the passage of an aircraft. To maintain level flight at constant speed, constant thrust is required; to climb or descend the aircraft whilst maintaining constant speed, the thrust must be increased or decreased; to increase or reduce the speed of the aircraft whilst maintaining level flight, the thrust must be increased or decreased.
In an aircraft, the thrust is generated in different ways according to the type of propulsion:
Turbojet: all the thrust is generated in the form of jet efflux from the rear of the engine. (Now used mostly in military aircraft).
Turbofan: most of the thrust is generated by a large fan at the front of the engine; a small percentage is generated by jet efflux.
Turboprop: most of the thrust is generated by the propeller; a small percentage is generated by jet efflux.
Piston: all the thrust is generated by the propeller.
The Power required to generate thrust depends on a number of factors, but in simple terms it may be said that the power is proportional to the thrust required times the aircraft speed.