The formal definition of a reef is a ridge of rock or shingle or sand at or just below the surface of the water.  Coral reefs fit this definition since they are composed of limestone rock and do not usually extend above the water surface. Coral reefs have living corals that live on the surface but mainly are non living remains that is rock (limestone).  The entire ecosystem is unique.  The term coral reef also encompasses the living animals and fauna in which they usually have some type of symbiotic relationship with.  The structure of the coral reef consists of skeletal remains that once established tend to grow upward and outward.  The life and death processes of the organism result in the steady contribution of their skeletal remains to the structure.  Tropical seas have a large amount of dissolved limestone that the corals use to produce and excrete their skeleton.  The death of each coral leave behind a skeleton in which the shape is species specific that governs the shape of the reef system.