Force

The concept of force was created to quantify the interaction between 2 objects.  Force is regarded as a vector because it has a magnitude and is exerted in a specific direction.  Force is most easily measured by utilizing a spring - when we stand on a spring scale, it is merely measuring the force of our weight due to gravity pressing against it.  By measuring how much a spring compresses when a force is applied we can obtain a very accurate measurement - an equation found (link to post) here.

Force is largely measured in newtons (N).  1 N is approximately the downward gravitational force of the Earth on a small apple (Newton formulated the idea of gravity by watching an apple fall from a tree, what a coincidence).

Force is the derivative of momentum, described as "the instantaneous time rate of change of the momentum of an object is equal to the net force acting on the object," or more simply, "the derivative of the momentum with respect to time is equal to the net force acting on the object."$$\frac{d\vec{p}}{dt} = \vec{F}_{net}$$

The net force ($\vec{F}_{net}$) is widely used in physics to describe all the forces acting on a system:  $$\vec{F}_{net} =  \sum\limits_{i=1}^n \vec{F}_i$$