Duchamp worked secretly on the piece from 1946 to 1966 in his Greenwich Village studio while even his closest friends thought he had abandoned art.
It is made of an old wooden door, bricks, velvet, twigs, a female form made of leather, glass, linoleum, and an electric motor. Duchamp prepared a "Manual of Instructions" in a 4-ring binder explaining and illustrating how to assemble and disassemble the piece.
It wasn't until 1969 that the Philadelphia Museum of Art revealed the tableau to the public.from: Wikipedia article
more info here (including diagram): Marcel Duchamp - Étant donnés: The Deconstructed Painting