When a body changes its velocity, it accelerates. When the velocity decreases, we usually say that it brakes.  
Acceleration is represented by a vector, so that to completely define the acceleration of a body you have to specify its module, direction and orientation.

The module of acceleration gives us an idea of how quickly its velocity varies.
If a body accelerates at 2m/s² it means that its velocity increases by 2 m/s every second.

The sign for acceleration depends on the Reference System we choose. So the acceleration of falling bodies (the acceleration of gravity or g=9,8 m/s²) can be written with a positive or negative sign: you decide which R.S. you choose.

To understand what acceleration is in more detail, it has to be strictly defined.