Corals are animals belonging to the phylum Cnidaria and are therefore related to other cnidarians like hydras, jellyfish, sea anemones, etc. They have a ring of tentacles, tipped with stinging cells, around a central opening that functions as both mouth and anus. With the tentacles the corals can defend themselves and capture zooplankton.

Corals can be solitary, though most corals are colonial, which means that each coral is made up of many individual polyps originating from one single individual. They can be divided into two main types, the hard corals (stony corals, or scleractinians) and the soft corals (gorgonians or octocorals).

As their names might suggest, these two types of corals have very different skeletal structures, and the corals that form reefs are predominantly colonial scleractinian corals. The scleractinian corals form their skeleton through deposition of calcium carbonate (CaCO3), and when the corals die the skeleton remains, adding new material to the coral framework.

Scleractinian corals grow in a number of different forms, depending on species and environmental conditions. The most common morphologies are massive (e.g. brain corals), branching (e.g. tabular corals, staghorn corals), encrusting (adhering to the substrate) and foliaceous (leaf-, or plate-like).

Most hermatypic (or reef-building) corals have symbiotic algae, zooxanthellae, in their cells. The zooxanthellae use the sun's energy to convert carbon dioxide from the seawater into energy-rich sugars and fats. Any excessive organic compounds that the algae don't need for their own respiration and growth are available to the host coral. In return, the zooxanthellae have a safe place to live within the coral tissue and the algae uses the coral's waste nutrients for growth.

The different colours seen on corals are due to the zooxanthellae and without the algae corals would be colourless.

Zooxanthellae -endosymbiotic algae

The clear waters surrounding tropical coral reefs are typically oligotrophic (nutrient-poor), yet these reefs sustain high productivity by supporting dense populations of marine organisms. This paradox is resolved by many coral reef invertebrates by accommodating unicellular, endosymbiotic algae (Symbiodinium spp., commonly referred to "zooxanthellae") within their tissues. Invertebrates with this kind of interaction include corals and giant clams. This photoautotrophic symbiosis allows for a beneficial exchange of nutrients between the algae and animal host. Organic carbon produced by the algal partner is released to the host for nutrition while inorganic metabolic wastes are recycled to fertilise algal photosynthesis.

Such algal-animal symbioses are especially common in the shallow-water environment of coral reefs where they are typically exposed to high levels of visible and ultraviolet radiation. On deeper parts of the reef there light intensity is lower, the capturing of zooplankton is more important than on shallower depth, there the amount of photosynthetic carbon translocated to the animal host is often sufficient to meet its energy requirement.

Since algal symbionts reside within coral tissues, overlying tissues of the animal host must be transparent to facilitate the penetration of the visible wavelengths of light required for algal photosynthesis. Without the algae, corals would for instance be colourless. When corals get stressed (for a number of different reasons) they can eject their zooxanthellae and become pale or turn completely white, a response known as coral bleaching. If the corals don't get hold of new zooxanthellae within approximately six months they die.