To answer this question accurately "what is barrier reef" we would have to say that barrier reef is coral reef or a series of coral reefs running parallel to the coast but separated from it by a deep channel of water. The Great Barrier Reef performs this function on a massive scale. For a distance of 2,300 kilometres on the Queensland coast and a futher 300 kilometres across Torres Strait the Great Barrier Reef provides protection for the Queensland coast from ocean swells coming from the Pacific Ocean.
The Cairns Great Barrier Reef Divesites are part of the world's largest coral reef system. It is composed of nearly 3,000 individual reefs and 920 islands stretching for over 2,650kilometres (1,620mi) over an area of approximately 345,000 square kilometres (133,100sqmi). The reef is located in the Coral Sea, off the coast of Queensland, Australia. This map courtesy of Catlin Seaview Survey shows the full extent of the barrier reef and how it meanders along almost the entire length of the Queensland coast:
The Great Barrier Reef is the world's biggest single structure made by living organisms and can be seen from outer space. The reef structure is composed of and built by billions of tiny organisms, known as coral polyps. It supports an incredible diversity of life and was declared a World Heritage Site in 1981. It is one of the seven natural wonders of the world.
The Great Barrier Reef is visited by millions of international tourists each year. Most people who want to see the reef depart on specialised vessel from Cairns to large viewing facilities located at strategic areas on among the hundreds of reefs that are accessibel from Cairns.
Snorkelling & diving
This is by far the best way to get a close up view of amazing corals, sea turtles, rays, sharks and tropical fish of every colour and size. Much of the diving and snorkelling on the reef is done from boats, however, there are also excellent reefs surrounding some of the resort islands.
Day trips by specialised reef boats leave from several places along the Queensland coast, as well as from island resorts, and normally stop at two or three different sites, allowing about 3 to 4 hours of time to snorkel, dive or just relax. Full day excursions to the reef cost around $250, including lunch, snacks and snorkel gear, with scuba diving an optional extra. Other coral viewing options such as underwater viewing chambers and glass bottom boats are also very popular. Most of the companies have a large moored pontoon with modern facilities for the comfort of customers. Cruises are available on a Cairns Day Dive Trip, a Port Douglas Great Barrier Reef Day Dive and a Airlie Beach Islands Diving
For certified divers or those who are unqualified but yearn to experience scuba diving, a live-aboard boat is the best option. Here you will have the option to dive up to 4 times a day, including night dives, and visit more remote parts of the Great Barrier Reef. Trip lengths vary from 2 to 7 nights, though 3 and 4 day trips are the most popular. Prices for a 3-day/2-night trip start at around $670 for a Cairns Liveaboard Dive Trip, and Airlie Beach Great Barrier Reef Dive Trips.
For a unique and spectacular view over the reef, take a helicopter or seaplane tour. Flights are available from Airlie Beach, with excursions taking in the spectacular Whitsunday Islands, and Cairns, with flights over rainforest-covered Green Island. A 30-minute scenic flight costs around $270. Flights are included in the Coral Sea Diving Liveaboard cruise on board Spirit of Freedom.