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".
A defect is something wrong with something, that stops it from being perfect. A defect in a machine might stop it from working at all.
A defect might also be called a flaw or imperfection.
You demolish something when you take it apart until there is nothing left.
When you say demolish, you usually mean a building being demolished.
A dent is a hollow, like a shallow hole in a surface. Dents are usually caused by hitting something. The car in the picture has a dent in its side.
You destroy something when you damage it so badly it is completely useless. You can destroy something by smashing it into tiny pieces, burning it or by dissolving it.
Dis forDestruction
Destruction is the act of destroying or demolishing something. It might also be used to describe the remains left once something has been destroyed.
Dis forDisintegrate
Something disintegrates when it breaks down into the components it was made from, or just breaks into small pieces. The building in the picture is slowly disintegrating.
A fissure is a relatively deep, narrow hole in something. Some fissures are natural features of things, others are caused by external damage.
A flaw is a quality of something that stops it from being perfect.
A flaw is also called a defect or imperfection.