Newton's 3rd Law of Motion states that every action force there is a reaction force that is equal in size, but opposite in the direction. The action and reaction forces can be looked at as "cause and effect". The action force is the force that is applied, whilst the reaction force is the force that occurs in response. An example is a ball bouncing up and down. The ball is pushes down on the floor with force, and the floor pushes the ball back up with the same amount of force.
