Subclass Alcyonaria (Octocorallia = eight tentacles)
Subclass Zoantharia (Hexacorallia = more than 8 tentacles)
Corals are colonial invertebrate animals that excrete a calcium carbonate skeleton (taxonomic class Anthozoa). They were made on the fifth day of creation along with all other aquatic organisms. Corals are known mainly as reef-builders, which provide habitats for a variety of other marine organism. There are two main types of corals: hermatypic, which produce reefs (hard corals) and are only found in tropical regions, and ahermatypic (soft corals), which do not produce reefs and are found worldwide.
The soft corals are made largely of a protein and calcium carbonate material, whereas the hard (stony) corals are made of calcium carbonate only without the protein. Examples of stony corals include: brain coral, star coral, staghorn coral, elkhorn coral, and pillar coral. The soft corals are not typically perceived as corals including the sea fans, sea whips, sea rods. The main difference between them is that the hard corals have an exoskeleton, while the soft corals lack the exoskeleton.
Most hermatypic (reef building) corals host symbiotic (living together) algae, which live inside their tissue. The algae are called zooxanthellae. Zooxanthellae give corals their color and need sunlight for photosynthesis. Thus, corals with zooxanthellae are only found above 30 meters in depth, black coral which do not have symbiotic algae grow in deeper water. They provide food to the coral and remove some of the corals waste products. In return the coral provides protection and access to light. 
Corals have all the different types some accordingly simpler than others. Corals are made of tiny organisms called polyps. For capturing its food, it uses a harpoon called a nematocyst, which is inside the mesentary filaments of the polyp The two cell layers that make up a polyp are called the epidermis and gastrodermis. Mesoglea is the tissue layer in between the both of them. The Corallite (the calcium carbonate that makes the body of the coral) is attached to the skeleton and deposited by one of the polyps. The Coral tissue that stretches atop the ground of coral in between polyps is the coenosarc. The name of the skeleton is the corallum. The septa is a plate-like structure that runs from the wall to the middle of the Corallite. The two versions of septa are insert septa and exert septa. The insert septum is positioned right below the Corallite wall. The exert septa is the opposite of the insert, which therefore makes it above the Corallite wall. 
There are two general types of coral known as imperforate and perforate. The imperforate coral's skeletons are solid where as the perforate coral's skeletons are more flexible or not as solid compared to the imperforate.
Corals have a single mouth and only exist in the polyp form, unlike many other cnidarians. The shape of the polyp is described as radially symmetrical. The mouth is surrounded by a ring of tentacles.
The corals can demonstrate either sexual or asexual reproduction. The Coral's way of expanding its population is by budding although it may differ when the new bud forms from the discs of the past polyp. Diploria is when the new polyp forms from the base of the old polyp. Fragmentation is the most common of the coral's asexual reproduction. A broken off piece can contain various individual polyps that can grow in another place if the environment is good. In Acropora Cervicornis, it is common for it to break off and start a new coral. All the corals of one species usually release their sperm at the same time within days of each other. Some release sacks of egg and sperm and others just the sperm. Fertilization takes place at the surface of the sea. The Zygote develops into larvae called planula which attach themselves to an appropriate hard surface and start their own population again.
The corals are different depending on what kind of habitat they live one them for example is Hawaii compared to the ones in the tropics. The Hawaii corals are young, small, and close to shore. As they begin to grow, they are known as fringing reefs and then barrier reefs. The corals are usually not the best in nutrients and lack of filter feeding animals for the other corals or sponges. The reefs in Hawaii are dominated by corals. There is not as many different species of corals in Hawaii as there are in the pacific. (Hawaii=40-Pacific=300) One way that all the corals differ is how far out they are from shore and how deep. The most important things are: sedimentation, salinity, and temperature. The main altering factor for the coral is its wave exposure. 
- General fact sheet Atlantic Acropora corals NOAA Fisheries. Accessed 27 November 2011.
-  Univ. Virgin Islands.
- Coral: animal, vegetal and mineral by Paula Weston. Creation Ministries International (CMI). Accessed 27 November 2011
- Marine Science by Genny Anderson. Accessed 27 November 2011
- Coral 101 Reproduction NOAA Coral Reef Conservation Program. Accessed 27 November 2011
- NOAA Coral Reef Conservation Program