Class Anthozoa -> Subclass Hexacorallia -> Order Scleractinia
Stony Corals of the Order Scleractinia are the architects of the reef structure. Scleratinia are characterized by having 6-fold symmetry as well as a calcium-based skeleton. 6-fold symmetry means the coral has six tentacles or tentacles in multiples of six.
The reef aquarium hobby long ago divided stony corals into two categories, Large Polyp Stony (LPS) and Small Polyp Stony (SPS). This view is somewhat antiquated, but still serves as a guideline for care where LPS require less light and less flow than their SPS counterparts. On the whole LPS are more tolerant to water quality than SPS, but MANY exceptions exist so by all means research prospective corals individually before purchasing any LPS or SPS corals.
Large Polyp Stony Corals are the builders of calcium carbonate reef structures found in the wild. The skeleton of these corals is slowly secreted by the epidermis at the base of each coral polyp. The rate that this occurs depends on the species. Some varieties of LPS such as Favites can grow very quickly while some massive LPS like Scolymia can take years to grow an inch. Large Polyp Stony Corals benefit from clean water and a consistent source of calcium, alkalinity, and magnesium.