Newton's Three Laws of Motion
Newtons three laws of motion govern the basis of how things move. At the basis of the three laws is the concept of force.
Newton's First Law of Motion
A rigid body subject to no net external force continues in a state of uniform motion.
No net force so uniform motion.
Newton's Second Law of Motion
A body of mass m subject to a constant net external applied force F continues in a state of uniform acceleration a: F=ma.
For the diagram above
Newton's Third Law of Motion
To every action there is an equal and opposite reaction.
Newtons Third Law implies that if you push something, it pushes back with an equal force. This does not mean that you are moved, since you may be standing on the ground. In this case the force will be transmitted through you to the Earth. The Earth will be pushed back.
Newron's Third Law implies conservation of momentum, since impulse is a change in momentum, and an equal and opposite force implies an equal and opposite impulse, so the net change in momentum is zero.