What's the difference between plants and coral?

For a long time scientists could not understand what kind of organisms corals were. Only in 1982, the French explorer proved that it was not sea plant. So, what is the difference between corals and usual plants? Corals are small sea animals which can exist alone or by some colonies. They can be called as multicellular organisms which have the forms of plants. That\'s why people usually think, they are plants. Corals can be of different shapes, forms and color. They eat small fish and prawns. So now we can surely say the corals are not plants.