"Acceleration" is a term used to describe a change in speed or direction. The term "speed" is used to describe an object when direction is unimportant, while the term "velocity" is used to describe an object that is changing direction.
Sir Isaac Newton discovered a number of basic principles that laid the foundation for classical mechanics. Newton's three laws of motion are used to describe the relationship between an object and the forces that act upon it when in it is in motion. According to The Physics Classroom, the second law of motion describes the basic principles of acceleration and velocity that affect all objects.