Corals are living organisms, belonging to the sessile marine cnidarians.

When many people see coral, their first reaction is that it is a plant or a rock on the seafloor, but coral is actually a living organism, and an animal at that. It belongs to the cnidarians, also called coelenterates, and is a close relative of jellyfish and sea anemones. The coral we usually see is often a colony formed by thousands of individual coral polyps gathered together; each polyp has a mouth, tentacles and a simple digestive structure, and actively preys on plankton in the water.

Coral has another important characteristic: it lives in symbiosis with zooxanthellae within its body. These single-celled algae live in the tissues of the coral polyps, providing nutrients to the coral through photosynthesis and also giving coral its vivid colors. Reef-building corals also secrete calcium carbonate, forming a hard calcareous skeleton, which accumulates over time into coral reefs, which is also why coral is often mistaken for stone.

Based on morphology and skeletal characteristics, corals can be broadly divided into two types: hard corals and soft corals. Hard corals have a hard skeleton and are the main builders of coral reefs; soft corals lack a hard exoskeleton, have soft bodies, and sway gently with the water currents. Reef-building corals mostly grow in warm, clear shallow seas, and because they depend on symbiotic algae, they need sufficient light to grow healthily.

Coral reefs are called the tropical rainforests of the ocean, providing habitat and breeding grounds for many marine organisms such as fish and shellfish, and they can also weaken waves and protect coastlines. Some species, such as Polycyathus corals, receive conservation attention because of their scarcity. Rising seawater temperatures, pollution and human destruction can easily trigger coral bleaching; after the symbiotic algae leave, coral will die if it cannot recover over the long term. Therefore, maintaining water quality and reducing destructive activities are key to allowing coral to survive.