Speed is a scalar. The displacement of an object divided by time . This means that whatever the direction of the displacement the velocity will have the same direction. The rate at which velocity changes is called acceleration. Acceleration is calculated as velocity divided by time .
Acceleration is a vector.
This whole set of difinitions is based on uniform acceleration. Speed, velocity, and acceleration are all time rates. For displacements negative and positive signs are vector directions. For accelerations the meaning of negative and positive depends on the sign of the velocity. If velocity is positive a positive acceleration means an increasing velocity and a negative acceleration means a decreasing velocity .