There are numerous species of coral, but they are broadly classified into two main categories – hard corals and soft corals. Hard corals have a hard exoskeleton made of calcium carbonate and form the backbone of coral reefs. Some of the common hard coral species include staghorn coral, brain coral, and elkhorn coral. Soft corals do not have a hard exoskeleton and are less structurally complex than hard corals. Some examples of soft coral species include sea fans, sea whips, and sea pens.