Velocity is the rate of change of position. The average velocity is the displacement Dx divided by the time interval over which it occurred,
The algebraic sign of indicates the direction of the motion.
On a graph of x versus t, the average velocity for the time interval Dt is the slope of the straight line connecting the points on the curve at the beginning and end of the time interval (see the figure).
Use the sliders to change ti and tf. 




On the other hand, the average speed
of the particle depends on the full distance it covers in a time interval Dt.