Forces are interactions between objects - a push or pull. Forces have the ability to make an object speed up, slow down, change direction or change its shape. Force is conventionally measured in units of newtons (N) or pounds (lbs).
Exerting a force over some distance represents a specific transfer of energy also known as work. Fields (electric fields, magnetic fields, gravitational fields) are spatial regions that exert forces on objects. The position of objects within these fields determine an object's potential energy. When an object is pushed into against the force of one of these fields, the object gains potential energy. An object that moves along the push of theses fields loses potential energy and gains kinetic energy (or thermal energy). Forces exchanging energy between potential and kinetic energy is an example of mechanical energy conservation (if thermal energy is increased then there's still energy conservation, but it's trickier).
Broadly, the forces are either fundamental forces or the everyday forces in our day to day life. Feel free to play around with the simulation below to find out more about how force affects motion and acceleration.
The University of Colorado has graciously allowed us to use the following PhET simulation. Explore out the PhET animation below to explore how forces change motion.