Once you have the concept of speed and velocity down acceleration is pretty easy to understand. If you recall, speed is the rate at which your moving. It's distance traveled divided by the time it takes to travel that distance. A natural question to ask is: what if your speed is changing? In that case we would need a term for the rate at which your speed is changing. The term we use is acceleration, which is just the change in an object's speed divided by the time it takes to make that change.
We had also discussed that velocity is the vector quantity which carries information about speed, but also direction of travel. In the same way we can think of acceleration as the vector that indicates both how the magnitude and direction of the velocity vector is changing. Conceptually, the important point here is that if an object is moving at constant speed, but in a changing direction (like a car moving around a gentle curve in the road) it is accelerating because its velocity is changing, even though its speed is constant.
© 2008 Kansas State University Physics Education Research Group.