What's the difference between speed and velocity?

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.