|MadSci Network: Physics|
You are absolutely right. Acceleration is defined as the change in velocity of an object in a unit of time. Its standard unit is metres per second per second. Deceleration (slowing down) is acceleration in the opposite direction to the object's motion.
Acceleration of a moving object is given by the gradient of its velocity-time graph; the steeper the slope the greater the change in acceleration. The gradient of a horizontal line is 0, the velocity is not changing and so at this point the object is not accelerating or decelerating.
Try the links in the MadSci Library for more information on Physics.