There are three major types of coral reefs that exist around the world, two of which are common to the Great Barrier Reef near Cairns. The types of reef are barrier reefs (hence the name of the Great Barrier Reef) and fringing reefs, both common to the Great Barrier Reef, as well as atoll reefs, which are rare.
Barrier Reef: Barrier Reefs are reefs that run parallel to a coastline but are separated from the coastline by a lagoon, or large body of deep, open water. They do not directly connect with the coastline, and instead have the ocean between them. Their highest points are close to the waters surface, which breaks bigger waves and protects the waters beyond them. They are also known for impeding ships as they are often to shallow to sail over (and have been known to cause shipwrecks). The Great Barrier Reef is the biggest example of a barrier reef in the world.
Fringing Reefs: Fringing Reefs are the most common type of reef. They are reefs that are found on the fringes of land, and grow directly from the shore. They surround island and continents, with Green Island and Fitzroy Island being the most common examples in the Cairns area. Fringing Reefs are also the type of reefs you find around the Whitsunday Islands, which are further south.
Atoll Reef: Atoll reefs are the most interesting of the three major types of reefs, with a very distinct identifier. They are circular reefs surrounding a deep lagoon, creating a ring in the ocean, and look very interesting if seen from the sky. They are often found in the middle of the open ocean and have a deep centre. They are formed when an island that once was surrounded by a fringing reef sinks into the ocean.
There are also ribbon reefs, patch reefs and platform reefs found on the Great Barrier Reef, all of which, put together, form the biggest single ecosystem on earth.