A Continuous force that casues all objects with mass to be pulled closer or pushed apart. Earth has more mass than the moon that is why we are on Earth but the sun has much more mass than Earth so that is why Earth orbits the Sun. Newton Theorized that the magnitude of an objects Acceleration is is equal to the distance of the orbit (circular Motion of earth Relative to the Sun) is constant of the speed to the Radius of the circular path.
Action Forces begin an objects movement. It also causes a Reaction Force.
Reaction forces are the effect of action forces and are important to Newtons third law of motion.