Acceleration is the rate at which a body changes its velocity and is also a vector quantity which needs a direction as well. It is measured in m/s/s or metres per second per second (metres per second squared - m/s2).
For example in the first 30m of a 100m sprint the sprinters velocity will be increasing and will have 'positive acceleration'. After they pass the line they will slow down during which time they will have 'negative acceleration' or decceleration.
Acceleration (m/s2) = Vf - Vi (m/s) / time (s)
Acceleration = Velocity final - Velocity initial divided by time.