Over time, the term MDF has become a generic name for any dry process fibre board. MDF is typically made up of 82% wood fibre, 9%urea-formaldehyde resin glue, 8% water and 1% paraffin wax.

This, and a lot more can be found at MDF

There are also low density and high density fiberboards, as well as medium.

MDF is used because it is structurally more stable than plywood or solid wood. MDF does not warp like plywood.

The major drawback with MDF is moisture/water damage. It can swell and disintegrate when wet. There are types of MDF that have more resins in the recipe to combat that drawback.

MDF should not be confused with OSB(oriented strand board), which is made with layers of wood chips or chip board(made with smaller chips of wood).

For all you greenies out there, they can make OSB and MDF from bushes; they don't need old trees.

The suppliers that I use have gone away from the formaldehyde based adhesive and instead have gone to a soy based adhesive called purebond. Thanks to the fine folks of California. None of what I have available here contains formaldehyde, but I can't speak for the rest of the country