Speed is the ratio of distance traveled to the time elapsed.
i.e. speed = distance/time
whereas, velocity is the ratio of displacement to the time elapsed.
i.e., velocity = displacement/time
Now, displacement can be 0, but distance traveled cannot be. Think about what happens when we throw a ball up in the air and it falls down. It has certainly traveled certain distance; however its displacement (difference between initial and final position) is zero (as the ball started from ground level and came back to it). Thus, in this case, velocity is zero, speed is not.
Also, speed is a scalar quantity, i.e. it only has magnitude. On the other hand, velocity is a vector quantity, since it has both the magnitude and direction.
Both of them are measured in the same units of length per unit time.
Hope this helps.
Speed is a scalar quanity that refers to "how fast an object is moving." Speed can be thought of as the rate at which an object covers distance. A fast moving object has a high speed and covers a large distance in a very short amount of time, while a slow moving object covers a small distance in a small amount of time.
In contrast, Velocity is a vector quanity that refers to "the rate at which an object changes its position. When referring to an object's velocity person must refer to the direction. For an example when describing the velocity of something a person must include both the speed and he direction such as a car traveling 55 miles per hour going east.
This is one of the essential differences between speed and velocity.