Newton's third law is: For every action, there is an equal and opposite reaction.

A force is a push or a pull that acts upon an object as a results of its interaction with another object. Forces result from interactions! Some forces result from contact interactions and other forces are the result of action-at-a-distance interactions.

According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces and are the subject of Newton's third law of motion.