A coral reef is a sea structure made out of living organisms which together form a wall. Stony corals are usually the main organism found in a coral reef, but other types of skeletal material also join together to form the thick walls. A coral reef is also "constructed" by the thousands of fish, sponges, and sea urchins grazing around the area. As these organisms move about, they actually help the coral reef settle and gain a more stable structure. Waves are also partly responsible for the size and format of a coral reef, as they affect the geology of the formation.

A coral reef is very effective at absorbing nutrients from the waters surrounding it. This includes obtaining phosphorus and nitrogen from sea waters, and other nutrients from small organisms making their home in the coral reef. A coral reef is one of the largest living communities in the world. Thousands of species make their home there, including over 4,000 different tropical fishes, jellyfish, crabs, sea urchins, and starfish. Mammals may visit a coral reef, but none actually live on or near it.

91.9 percent of the world's coral reef mass is situated in the Indo-Pacific region, with Australia taking a major percentage of that. There are basically no coral reef structures on the American west coast or on upper Asia, including Russia. This is mainly due to cold waters, but also to the effect of powerful water currents in the area.

Humans are the biggest threat to the integrity of a coral reef. Because of pollution, boat traffic, and aggressive fishing, coral reefs are now considered in danger and are protected under the Great Barrier Reef Marine Park Authority, an Australia-based institution. The illegal trade on saltwater aquarium fish is also a major factor on coral reef destruction, as little can be done to stop collectors from around the world to request exotic, pricey species.

A coral reef is also susceptible to major environmental changes, including global warming, which affects water currants and marine migration. Coral bleaching, a form of erosion, occurs when water temperatures reach higher-than-normal levels. This can prove disastrous for a coral reef.