Contact force is defined as the force exerted when two physical objects come in direct contact with each other. Other forces, such as gravitation and electromagnetic forces, can exert themselves even across the empty vacuum of space.
The concept of force was originally defined by Sir Isaac Newton in his three laws of motion. He explained gravity as an attractive force between bodies that possessed mass (gravity within Einstein's general relativity doesn't require force).
Force is a vector. The SI unit for force is the newton (N). One newton of force is equal to 1 kg * m/s2.