Coral is the human standard of beauty turned inside out: the polyps live outside and secrete their white endo-skeletons in every color and shape imaginable. The animal itself, the polyp, usually feeds at night, keeping its waving tentacles withdrawn during the day. This is a defense against predators and ultraviolet light; nighttime feeding also prevents shading of the light-loving algae.
Corals often fall into categories of "hard" (the spiny and solid-looking structures) and "soft" (such as the gently waving sea fans and leather corals). But all these large, visible structures are only thin layers of living tissues outside the creatures' skeletons.