Australia’s Great Barrier Reef is primarily classified as a barrier reef.
A barrier reef generally runs roughly parallel to a coastline but is separated from land by a lagoon or other broad stretch of water. The Great Barrier Reef fits this broad definition because its offshore reefs and islands form a long system outside Queensland’s mainland coast, with substantial inshore waters between the coast and many reef structures.
The Great Barrier Reef is not one uniform wall of coral. It includes many reef forms, including fringing reefs, platform reefs, patch reefs, ribbon reefs, and cays. “Barrier reef” describes the overall arrangement of the system, while individual reefs within it may have different shapes and ecological conditions.
An atoll is a different geomorphological form, usually a ring-shaped reef surrounding a lagoon where a volcanic island has subsided. Although some Great Barrier Reef reefs may look ring-like, the whole system is not an atoll.