Speed is the rate of motion, or the rate of change of position. It is expressed as distance moved (d) per unit of time(t). Speed is a scalar quantity with dimensions distance/time. Speed is measured in the same physical units of measurement as velocity, but does not contain an element of direction. Speed is thus the magnitude component of velocity. Velocity contains both the magnitude and direction components.
The difference between speed and velocity is that velocity is speed with a direction vector associated with it. It's that direction vector that makes them different. Let's back up a bit and review.
Speed is displacement (distance traveled) per unit time. Velocity is displacement (distance traveled) per unit time in a given direction. The "directionallity" associated with velocity makes it a vector quantity, while speed is a scalar quantity.