Newton's Second Law describes the change in motion an object with mass experiences when a force is applied. This law is describe mathematically by a simple equation.
F = ma
Where F is the force applied, m is the mass of the object, and a is the acceleration experienced by the object. Acceleration is defined as a change in motion over time, and physicists lump both an increase in speed (acceleration), and a decrease in speed (deceleration) into their definition of acceleration. In this definition, a deceleration will result in a negative force. This seems a little odd, until you think of a force as either a push or a pull. If the result is negative, the force is in the opposite direction. Forces are vectors. A vector is a quantity that has both a direction and a magnitude (value). Acceleration is also a vector, but mass is not.
Newton's Second Law also means that if a constant force is applied to an object, resulting in motion, that object will accelerate. If the force is then removed, the object will maintain a constant speed unless otherwise acted upon by another force (Newton's First Law). Often, more than one force will be acting on an object at one time. It is the sum of these forces that ultimately determine the acceleration.
Multiple forces can act on an object from different directions. This affects the sum of the forces (net force). If two forces of equal magnitude are acting on an object in exactly opposite directions, the sum of the forces is zero. If two forces of equal magnitude act on an object in the same direction, the sum of the forces is double one of the forces. Mostly though, forces aren't equally opposite or equally the same. The net direction of the net force is determined by the angles between the directions of the different forces. Calculating the net force requires a little bit of trigonometry. This is simple enough in a two dimensional system, but gets more complicated in three dimensions.
It is also important to note that the mass of an object never changes no matter where it is in the Universe. Mass should not be confused with weight, which is actually the force experienced by a mass when acted on by acceleration due to gravity.
