Coral Reefs

Coral reefs are aragonite structures produced by living organisms, found in shallow, tropical marine waters with little to no nutrients in the water. High nutrient levels such as that found in runoff from agricultural areas can harm the reef by encouraging the growth of Algae. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate (limestone). The accumulation of skeletal material, broken and piled up by wave action and bioeroders, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life. Although corals are found both in temperate and tropical waters, reefs are formed only in a zone extending at most from 30°N to 30°S of the equator; the reef-forming corals do not grow at depths of over 30 m (100 ft) or where the water temperature falls below 22 °C (72 °F).

Coral reefs can take a variety of forms, defined in following;
Apron reef — short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore.
Fringing reef — reef that is directly attached to a shore or borders it with an intervening shallow channel or lagoon.
Barrier reef — reef separated from a mainland or island shore by a deep lagoon; see great Barrier Reef.
Patch reef — an isolated, often circular reef, usually within a lagoon or embayment.
Ribbon reef — long, narrow, somewhat winding reef, usually associated with an atoll lagoon.
Table reef — isolated reef, approaching an atoll type, but without a lagoon.
Atoll reef — a more or less circular or continuous barrier reef surrounding a lagoon without a central island; see atoll.
Bank Reef — Bank reefs are larger than patch reefs and are linear or semi-circular in outline.

(All info from Wikimedia Inc.)