"For every action, there is an equal and opposite reaction."
If one object X applies with a force to another object Y, the object Y will also exert an equal and opposite force to the object X. The forces always occur in pairs with same magnitude but opposite direction.

Figure 10:
Source: https://road2animate.wordpress.com/2010/06/26/laws-of-motion-useful-knowledge-for-animators

Example: Shooting with a rifle is a typical example for explaining the Newton's Third Law. When the shooter fires his rifle, he experiences sudden recoil of the rifle. This "kick" felt by the shooter is a reaction force which acts upon the rifle. This force is equal in magnitude to the force that pushes forward the bullets.


Figure 11:
Source: https://philipphysics.wikispaces.com/Newton%27s+Third+Law