What are Reefs?
Reefs can be defined in two separted but interelated ways, according to the view, thus we can distinguished between a bioogival and a geologicval concept of reef as followows:

Biological Reef:
Are the most densely populated environment of the sea floor where calcareous skeletons of many animals and algae build mounds. Optimal growth conditions occur in the warm, shallow. lighted (Photic) zone with oxygenation of the water and abundant nutrients. Here we distinguish three types of reefs:

(a) Fringing reefs: grow directly adjacent to shore

(b) Barrier reefs: are separated from shore by a shallow shelf

(b) Atolls: are rings of reefs with a central shallow lagoon. They are a ring-shaped group of islands made of coral reef or coral sands built in a submerged volcano

At present most reefs grow only in shallow sea between 30 N and 30 S

Geological reef
Geologists define reefs and related terms (for example, bioherm, biostrome, carbonate mound) using the factors of depositional relief, internal structure, and biotic composition. There is no consensus on one universally applicable definition.

A useful definition distinguishes reefs from mounds as follows. Both are considered to be varieties of organosedimentary buildups: sedimentary features, built by the interaction of organisms and their environment, that have synoptic relief and whose biotic composition differs from that found on and beneath the surrounding sea floor. Reefs are held up by a macroscopic skeletal framework. Coral reefs are an excellent example of this kind. Corals and calcareous algae grow on top of one another and form a three-dimensional framework that is modified in various ways by other organisms and inorganic processes. By contrast, mounds lack a macroscopic skeletal framework.

Mounds are built by microorganisms or by organisms that don't grow a skeletal framework. A microbial mound might be built exclusively or primarily by cyanobacteria. Excellent examples of biostromes formed by cyanobacteria occur in the Great Salt Lake of Utah (USA), and in Shark Bay, Western Australia.