What is the difference between an atoll and a reef?

What is the difference between an atoll and a reef?

An atoll is a ring-shaped coral reef or a series of coral islands surrounding a lagoon, typically formed on the rim of a submerged volcano. On the other hand, a reef refers to a ridge or a mound of rock, sand, or coral that lies at or near the surface of the ocean. While both atolls and reefs are formed by coral, the main difference lies in their shape and formation.