1. Weight is the force of gravity. It acts in a downward direction—toward the center of the Earth.
2. Lift is the force that acts at a right angle to the direction of motion through the air. Lift is created by differences in air pressure.
3. Thrust is the force that propels a flying machine in the direction of motion. Engines produce thrust.
4. Drag is the force that acts opposite to the direction of motion. Drag is caused by friction and differences in air pressure.
When an airplane is flying straight and level at a constant speed, the lift it produces balances its weight, and the thrust it produces balances its drag. However, this balance of forces changes as the airplane rises and descends, as it speeds up and slows down, and as it turns.
A spacecraft has weight, even in orbit, and uses thrust to reach space and to maneuver. But lift and drag—both created by movement through air—are absent in the near vacuum of space.