Fire corals are colonial marine organisms that look rather like real coral. However they are technically not corals; they are actually more closely related to jellyfish and other stinging anemones. They are members of the phylum Cnidaria, class Hydrozoa, order Capitata, family Milleporidae.

Fire corals have a bright yellow-green and brown skeletal covering and are widely distributed in tropical and subtropical waters. They appear in small brush-like growths on rocks and coral. Divers often mistake fire coral for seaweed, and accidental contact is common. Upon contact, an intense pain can be felt that can last from two days to two weeks. The very small nematocysts on fire corals contain tentacles that protrude from numerous surface pores (similar to jellyfish stings). In addition, fire corals have a sharp, calcified external skeleton that can scrape the skin.

Fire coral has several common growth forms; these include branching, plate and encrusting. Branching adopts a calcerious structure which branches off, to rounded finger-like tips. Plate adopts a shape similar to that of the smaller non-sheet lettuce corals; therefore erect, thin sheets, which group together to form a colony. The latter; "encrusting", is where the fire coral forms on the calcerious structure of other coral or gorgonian structures.