Though Newton's law states that a body in motion stays in motion, in real life you know that a moving object eventually slows down. This is due to the force caused by friction, which happens when the molecules of two objects briefly form bonds between each other as they move past one another. Overall, these interactions slow the movement of the object, which is a change in the velocity.
A spinning object follows a curved path, which means its direction is constantly changing from the straight line it would follow without application of an external force. The inward external force applied is known as centripetal force, not to be confused with centrifugal force, which is an outward force. An example is whirling an object on a string. The string provides the centripetal force that allows the object to continuously change its velocity to follow a curved path.
Gravity is an external force which applies to falling objects. Falling objects fall at an increasing velocity, up until their terminal velocity, which is the limit provided by drag from the air. Before then, the change in velocity is caused by the acceleration due to gravity.
Though you may hear the word "deceleration," in physics, there actually is no such word for a decrease in velocity. Instead, a decrease in velocity is also known as acceleration, since it is still a change in velocity. Applying the brakes in your car applies an external force to your spinning wheels, and the resulting slow-down is most properly called acceleration.