A coral reef is composed of thousands of coral polyps, which is a tiny animal, with a circle of tentacles around its mouth, that grow in colonies. The reef can be hundreds of miles long. Not all corals are hard, some are soft and look like trees and bushes which gives the coral the ability to bend with the movement of the water.
Jonathan Bird, "Wonders of the Seas: CORAL REEFS," Oceanic Research Group, 2013, http://www.oceanicresearch.org/education/wonders/coral-reefs.htm.