Something is decaying if it is slowly rotting or otherwise degrading and breaking down.
A civilization that is declining may be said to be in decay, for example, "the decay of the Roman empire".
Dis forDeck (structure)
Dis forDeck (cards)
A full set of cards, as purchased from a shop in a box, is called a deck or pack.
Dis forDeck chair
Dis forDecoration
Something is there for decoration if it is there to look good, or to make the thing it is attached to look good. The picture is of some decorative trim on a building.