Speed is the magnitude of the rate of change of an object's position with time or the magnitude of change of its position per unit of time is the speed of an object in everyday use and kinematics; it is thus a scalar number.
Like this car that's traveling 30 miles per hour.
The rate of change of an object's position with regard to a frame of reference is its velocity, which is a function of time. A definition of an object's speed and direction of travel.
Like I'm walking 1 miles up north.
The rate of change of an object's velocity with respect to time is called acceleration in mechanics. Accelerations are measured in terms of vectors. The orientation of the net force acting on an object determines the orientation of its acceleration.
Like I'm starting off slow skating and starting to skate quicker.