What is the difference between speed and velocity?

Speed and velocity may seem like identical measures but this is not the case. Speed is a scalar measurement meaning it only takes into account magnitude not direction, velocity is a vector quantity and therefore describes speed in a specific direction. For example:

If a car is moving along a road, which we take as the positive direction, at 5 ms^-1 then it has a velocity of 5ms^-1 and a speed of 5ms^-1 . However if the car was moving backwards along the road at 5ms^-1 it would have a speed of 5ms^-1 but a velocity of - 5ms^-1.

As scalar quantities are only concerned with magnitude not direction they can't be negative.