Hardboard - commonly referred to as Masonite which historically is a different product - is a High Density Fibreboard (HDF) product constructed from timber fibres and is uncoated ready to accept surface veneers or coatings. Generally the top face is smooth, while the back face has a matted texture.
Tempered hardboard has been coated with a thin film of linseed oil and then baked; this gives it more water resistance, impact resistance, hardness, rigidity and strength.