When a body increases its speed, it accelerates. When the speed decreases, we say the body is braking. 

Acceleration is represented by a vector; so that, to completely define it you must specify its point of application, its module, the  direction and the orientation.

   - The point of application is in the body's centre of gravity.
   - The module of acceleration gives us an idea of how quickly its velocity     varies. If a body accelerates at 2m/s² it means that it increases its velocity     at 2 m/s each s.
   - The direction of the acceleration vector coincides with the direction of     the vector of the increase in velocity, which does not have to coincide with     the direction of the movement.
   - The orientation of the accleration vector coincides with the orientation     of the vector of the increase in velocity.

The sign for the acceleration vector depends on the reference system which we choose. So for the acceleration of falling bodies (the acceleration of gravity, or g=9.8 m/s²) we can use a positive or negative sign: you choose the most suitable R.S. for you: the window from which the object is dropped or the ground where it will land. The result of the exercises will not vary.