The three fundamental laws defining the relationship between an object's motion and the forces acting upon it.
Newton's First Law states that an object will remain at rest, or continue to move in a straight line at a constant velocity, unless acted upon by a net external force.
When a car brakes abruptly, you lurch forward. Your body is attempting to maintain its constant forward velocity because no net horizontal force was immediately applied directly to you.
Newton's Second Law describes how the velocity of an object changes when it is subjected to an external force. The acceleration of an object is directly proportional to the net force applied, and inversely proportional to its mass.
More massive objects require greater force to achieve the same acceleration.
For every action, there is an equal and opposite reaction. When two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction.
Even though the forces are strictly equal, the accelerations of the two objects may be vastly different if their masses differ, as required by the Second Law $(a = F/m)$.