The short answer is that velocity is the speed with a direction, while speed does not have a direction.
Speed is a scalar quantity—it is the magnitude of the velocity. Speed is measured in units of distance divided by time (e.g., miles per hour, feet per second, meters per second, etc.).
Velocity is a vector quantity—when giving the velocity we must specify the magnitude (the speed) and the direction of travel. For example you might drive 100km/hr (the speed) in a northerly direction.
This is analogous to the difference between distance (a scalar quantity) and displacement (the distance with direction).