Newton's First Law of Motion
Newton's first law of motion states that an object at rest will stay at rest until acted upon by a nonzero net force. An object moving at a constant velocity will continue moving at a constant velocity unless upon by a nonzero net force. If an object isn't moving it won't start moving unless a force acts on it. If an object is moving it will continue at a constant velocity until a force acts to change its speed or direction.
Inertia
All objects, moving or not, resist changes in motion. Inertia is the resistance to motion. Newton's first law of motion is also called the law of inertia. Inertia explain many common events, including why you move forward in your seat when the car you are in stops suddenly. You keep moving forward because of inertia. Some objects have more inertia than other objects. The greater the mass, the greater the inertia, and the greater force required to change its motion.