Medium density fiberboard is a product made of wood fibers as opposed to the wood chips particleboard is made of. The fibers are almost as fine as cotton. These fibers are then mixed with resins and possibly other additives that can make it fire resistant, moisture resistant, or formaldehyde free. The advantages of MDF over particleboard are that it machines better, less paint is absorbed when applied, it is smoother, it lays more flat, and there is less chipout on the edges. The advantages MDF has over real wood are that there are no defects in it, it is cheaper, it machines better, and it is more stable. The disadvantages MDF has when compared to particleboard is that it is more expensive and heavier. The disadvantages when compared to real wood is that it is heavier and it doesn't have the screw strength hold real wood has.

There are different densities of fiber board available. The amount of resin content and the density by which it is pressed together determines this density. HDF, or High Density Fiberboard, is used typically in speaker box construction because of it's ability to reflect sound waves better than a MDF would allow. 48# density is typical of MDF and 55# is typical of HDF.