Corkboard is a board made out of compressed cork. It is light-weight and not very strong, but it is a good, cheap insulator. It insulates not only from the heat and cold, but also from sound and mechanical vibrations.
It was invented by John T. Smith of Brooklyn, NY. He received his patent for corkboard (or, more specifically, a new "Process of Treating Cork") on July 14, 1891. It was originally used mostly for the insulation of cold storage areas.
Corkboard is made from lower quality cork (virgin and secondary cork; see the cork node) than are wine corks and other products that are made whole, punched directly from the cork bark. This lower quality cork has to be compressed under high temperatures in order to hold together. This process also causes the cork granules to darken in color and expand.
'Corkboard' also refers to rectangles of corkboard that have been cut and framed for the specific purpose of being hung on the wall and having things pinned to them, AKA a bulletin board. These are close relatives to whiteboards and blackboards, except that it is hard to pin anything to a blackboard, or write anything on a corkboard.
Often written as two words, cork board.