Hardboard (not to be confused with hardwood), also called high-density fiberboard (HDF), is a type of fiberboard, which is an engineered wood product. This product is also known under Isorel or Masonite brand names.
It is similar to particle board and medium-density fiberboard, but is denser and much stronger and harder because it is made out of exploded wood fibers that have been highly compressed. Consequently, the density of hardboard is 31 lbs or more per cubic foot (500 kg/m³) and is usually about 50-65 lbs per cubic foot (800–1040 kg/m³). It differs from particle board in that the bonding of the wood fibers requires no additional materials, although resin is often added. The hardboard variety is made without resin. The HDF version is made with resin. Unlike particle board, it will not split or crack.