Coral reefs are made up of many different type (species) of corals. There are two main groups of corals: HARD coral, which has a hard outer skeleton made of calcium carbonate and SOFT coral which has small amounts of calcium carbonate embedded inside it. Although corals come in many different colors and sizes they are all made up of tiny animals known as POLYPS. A polyp looks very similar to a jellyfish turned upside down.
In a HARD coral, the polyp builds a calcium carbonate cup around itself. Many of these cups are cemented together to make up a coral colony. Coral reefs are formed, when hundred of these hard coral colonies grow next to, and on top of each other. Most species of coral polyps retract into their calcium carbonate cups during the day. Therefore many people regard corals as inanimate rocks. However, at night the polyps emerge and wave their tentacles about to catch tiny creatures called plankton. Corals also have within their structure a special types of algae called zooxanthellae (a one celled plant). Zooxanthellae use sunlight and carbon-dioxide to produce energy or oxygen and other nutrients (a process known as photosynthesis). The relationship between the coral polyp and zooxathellae is a symbiotic relationship, which means that both parties mutually benefit from the association. In the association, the polyp provides shelter for the zooxanthellae, while getting part of its food synthesized by zooxanthellae and oxygen.