Question

what is speed classified as a scalar quantity and velocity classified as a vector quantity?

Answer 1

Vectors have speed AND direction.
Speed is classified as a scalar quantity because it only has magnitude (numerical value and unit of measurement) such as 50 mi/h.
Velocity is classified as a vector quantity because it has magnitude and direction, 50 mi/h north. By including direction, you are giving more information than a scalar quantity (requiring one unit).
It's just the way we define it.