Corals are marine cnidarians (Phylum: Cnidaria; Class: Anthozoa) existing as small anemone-like polyps, typically forming colonies of many individuals. The group includes the important reef builders known as hermatypic corals, found in tropical oceans, and belonging to the Subclass: Zoantharia, Order: Scleractinia (formerly Madreporaria). The latter are also known as stony corals inasmuch as the living tissue thinly covers a skeleton composed of calcium carbonate. The hermatypic corals obtain much of their nutrient requirement from symbiotic unicellular algae called zooxanthellae, and so are dependent upon growing in sunlight. As a result, these corals are usually found not far beneath the surface, although in clear waters corals can grow at depths of 60 m (200 ft). Corals breed by spawning, with all corals of the same species in a region releasing gametes simultaneously over a period of one to several nights around a full moon.

There are several other types of corals, notably the octocorals (Subclass: Octocorallia) and corals classified in other orders of Subclass: Zoantharia: to wit, the black corals (Order: Antipatharia) and the soft corals (Order Zoanthinaria). Most other anthozoans would be treated under the common name of anemone.

Corals are major contributors to the physical structure of coral reefs that develop only in tropical and subtropical waters, but corals exist even in cold waters, such as off the coast of Norway. The most extensive development of extant coral reeflik is the Great Barrier Reef off the coast of Queensland, Australia.

Corals in southern California

Various coral skeletons in a zoological display.

Coral can be sensitive to environmental changes, and as a result are generally protected through environmental laws. A coral reef can easily be swamped in algae if there is too much nitrogen in the water. Coral will also die if the water temperature changes by more than a degree or two and becomes too hot or too cold or if the salinity of the water drops. In an early symptom of environmental stress, corals expel their zooxanthellae; without their symbiotic unicellular algae, coral tissues are colorless, revealing the white of their calcium carbonate skeletons, an event known as 'coral bleaching'. A combination of temperature changes, pollution, and overuse has led to the destruction of many coral reefs around the world. This has increased the importance of Coral Biology as a subject of study.

Ancient coral reefs on land are often mined for limestone.

Reddish coral is sometimes used as a gemstone especially in Tibet. Pure red coral is known as 'fire coral' and it is very rare because of the demand for perfect fire coral for jewellery-making purposes.

Coral is also a color, a pinkish orange, named after the above cnidarians. On a browser that supports visual formatting in Cascading Style Sheets, the following box should appear in this color: