What is force?
1)  Force is a push or pull
2)  Force is the capacity to do work or cause physical change
3)  Force= Mass times acceleration (F = ma)
4) A force is that which changes or tends to change the state of rest or motion  of a body.
Examples:
For  simplicity sake, all forces (interactions) between objects can be placed into  two broad categories: contact forces, and forces resulting from action-at-a-distance.
Contact  Forces include: frictional forces, buoyant forces, normal forces, and air  resistance forces
Action-at-a-distance  forces include: gravitation, electrostatic and magnetic forces.
Measuring  Force:
Force  is measured using either the English System of Measurements or the International  System of Units (SI).
Common  Units of Force
--->>SI:  Newton (N) 1 N = 0.225 lb;
One  Newton (N) of force is defined as the amount of force needed to accelerate 1 kilogram  (kg) of mass at a rate of 1 meter per second squared (m/s2).
1  Newton = 1 kg m/sec2 (A kilogram is the amount of weight at which 1 N of force  will accelerate at a rate of 1 m/s2.)
--->>English  System: Pound (LB) 1 LB = 4.448 N
In English system of measurements, a slug is the amount of mass that 1 pound of  force will accelerate at 1 ft/s2, and a pound mass is the amount of mass that  1 LB of force will accelerate at 32 feet/s2.
Describing  a Force:
A  force is a vector  quantity . A vector quantity is a quantity which has both magnitude and direction.  To fully describe the force acting upon an object, you must describe both its  magnitude and direction. Thus, 10 Newtons of force is not a complete description  of the force acting on an object. 10 Newtons, downwards is a complete description  of the force acting upon an object.
(Note:  What is the difference between vector  and scalar quantities ? A vector has both strength and direction, a scalar  quantity can be described using only 1 quantity, magnitude. Examples of scalar  quantities are: time, energy and volume since they only represent magnitude and  no direction.
What  is the Difference between Mass and Weight?
Shown  below are two types of scales commonly used in the classroom --a spring scale  (left) and a simple balance beam scale on the right.
On  earth the spring scale reads 100g with an unknown mass attached at the bottom.  To balance the scale on the right a 100g mass was also needed.
If  we were to take both scales to the moon, what would the the spring scale read?  How much mass would be needed to balance the 100g mass on the balance beam? Can  you explain your answer? For more details see the Mass  and Weight Page.