If a Force is applied to a body, it will cause that body to move. A body that is already moving will change its speed or direction. Note that the term 'change its speed or direction' implies that an acceleration has taken place.
This is usually summarised in the formula;
F = ma
Where F is the force, m = mass of body and a = acceleration.
The units of force should be kg.m/s2 but this is replaced by the Newton.
Hence, "A Newton is the unit of force that when applied to a mass of 1 kg. causes that mass to accelerate at a rate of 1 m/s2.
Forces can also cause changes in shape or size of a body, which is important when analysing the behaviour of materials.