This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
MDF (Medium Density Fiberboard) is a hardboard product comprised of hot
compressed wood fibers glued together. The color of this material can
vary from light to dark brown depending on the brand of manufacturer.
Particle Board or Flakeboard consist mostly of recycled wood chips. The
size of the wood chips used varies which means that the coarser the wood
chips reduces the structural density of the material.