Question

what is the main difference between speed and velocity ?

Answer 1

SPEED: it is the distance travelled per unit time. 
speed = distance/time 
it is a scalar quantity 

VELOCITY: it is the shortest distance travelled by a body per unit time 
velocity = displacement/time 
it is a vector quantity

Answer 2

speed is calculated using the distance and  velocity is calculated using displacement.
speed is the distance travelled per unit time and velocity is the shortest distance travelled per unit time.