What is it?

Newton's Third Law of Motion states that when two particles interact (or have a collision), the force exerted by the first particle on the second is equal and opposite to the force exerted by the second particle on the first.

Newton's Third Law of Motion is usually simply stated as "For every action, there is an equal and opposite reaction."

Give me an example!

Have you ever watched a game of billiards? When one ball strikes the other (action), it meets with a force from the other ball (reaction). After the collision, the second ball is set in motion and the first ball slows down or stops, doesn't it? The momentum of the first ball is transferred to the second ball, and vice-versa.