MDF is a wood based composite, that is, it is made from wood fibres, which are glued under heat and pressure, as with hardboard.
Different to plywood or chipboard, MDF is dense, flat, and very smooth, and because of this perfect smooth finish, it can be painted without preparation, resulting in a quality finish. It can also be varnished, stained or finished with a laminate or veneer.
MDF can be used for virtually any internal woodworking project where ordinary wood could be used. It's smooth surface texture and lack of internal grain structure means MDF can be machined in all directions, and can be cut or machined to any shape where ordinary wood would have restrictions, and it lends itself very much to the router. It is also widely used in the molding and decorative timber business, and in furniture making trade. It can be bonded to itself or timber using ordinary PVA adhesive.
Unfortunately, using MDF does have a down side, as it can be dangerous in use due to the substance called urea formaldehyde, which is released from the material when being machined, sawn, or sanded, and safety precautions such as good ventilation, face masks and goggles should be used at all times.
MDF is made in standardised 2440 mm x 1220 mm sheet size, in thickness sizes of 6mm, 9mm, 12mm, 18mm, and 25m. It is also available in DIY stores in smaller more convenient sizes.