MDO, or medium density overlay, it's engineered plywood with a resin treated fiber applied to both faces. This gives an extremely smooth finish that is easy to paint. In addition it's rated for exterior use, which means it is safe from the elements, insects, or other outdoor hazards, making it a perfect choice for the new deck or porch or even teardrop !!
http://www.bobvila.com/TipLibrary/Subject/Framing/Wood/0299-MDO_Plywood.html
It weighs 75Lb for a 8 X 4 X 3/4" thick Sheet, so the weights only fractionaly higher.
Paint finishes on MDO are up to three times more durable than the same finish applied to ordinary plywood.Ideal for Painted Signs: In many instances, overlaid MDO plywood has proven a more durable sign material than metal. MDO plywood is resistant to the elements with no danger of rust or corrosion.
Edit to add sheet thickness to weight