Fringing reef; barrier reef; atoll কী?

A fringing reef is a reef attached to the shore of an island, with no lagoon, or only a narrow, shallow channel, between them. A barrier reef is a reef separated from the shore by a lagoon, on the flanks of an island that has sunk while the reef grew upward. An atoll is a ring of reef, with low islands of reef debris, around a lagoon with no volcanic rock above sea level: its volcano lies entirely under the reef.