Here's some information I was able to look up.
MDF stands for Medium Density Fiberboard and MDO stands for Medium Density Overlay.
MDO is more similar to plywood, with a waterproof paper covering on it. It is denser than plywood and more stable (with respect to moisture) than MDF is.
MDF is made of what they call wood flour , a very fine powdery sawdust material, then compressed under an enormous amount of pressure , so as in particle board there is no possibility of any core voids. Be careful using MDF; as it has many chemicals within......use your respirator....not a dust mask.
Here's a website that might help you understand it a little better.http://www.ehow.com/way_5841195_can-subwoofer-box-out-plywood_.html?ref=Track2&utm_source=ask