Acceleration is a vector quantity, which means that it has both a direction and a magnitude. Acceleration is the rate at which the velocity of an object changes. Velocity is also a vector and also has a direction.
The directions of an object's velocity and acceleration do not have to be the same. For example, if a car has an initial velocity of 25 km/h and is slowing to 10 km/h, the velocity vector still points forward and the acceleration vector points backward. Negative acceleration usually means that the object is slowing down, whereas positive acceleration typically means that it is speeding up.
The average acceleration of an object is determined by calculating the difference in velocity and dividing it by the time it took for the velocity to change.