A force is an interaction between objects that tends to produce acceleration of the objects.
A force is a vector, with both a magnitude and a direction.
The MKS unit of force is the newton (N). 1 N = 1 kg m / s2.
Acceleration occurs when there is a net force on an object; no acceleration occurs when the net force (the sum of all the forces) is zero. An acceleration produces a change in velocity (magnitude and/or direction), so an unbalanced force will change the velocity of an object.
Isaac Newton (1642-1727) studied forces and noticed three things in particular about them. These are important enough that we call them Newton's laws of motion.