The concept of force derives from Newton’s laws, which describe force as the quantity that causes changes in motion of physical objects. In other words, forces change momentum of physical bodies, makes them accelerate or rotate.
According to Newton’s first law, if there are no external forces acting on an object, it will maintain its state of motion, i.e. if it is staying still it will continue to stay still and if it is moving, it will continue to move in a straight path with constant velocity.
There are 4 fundamental forces recognized in Physics:
- Gravitational force
- Electromagnetic force
- The strong interaction
- The Weak interaction
However, in the scope of Newtonian mechanics, only electromagnetic and gravitational forces are dealt with. The last two forces are confined to very small length scales(~10-15 meters).
Forces are vector quantities.
The concept of force as defined in Newton’s laws break down in the scope of Quantum Mechanics and Relativity. Relativity treats gravity as curvature of spacetime and Quantum Mechanics treats force “carriers” as entities with both particle and wave like properties.
For details, see an excellent video series by Scishow here