Corals are invertebrate animals belonging to the phylum Cnidaria (from the Greek word 'Cnidos' meaning stinging nettle). There are five main classes of Cnidaria: Anthozoa, Hydrozoa, Cubozoa, Scyphozoa and Staurozoa.

Species referred to as corals occur within in the Anthozoa and Hydrozoa. They can exist as individuals or in colonies. Many secrete calcium carbonate external skeletons.