The Great Barrier Reef is the world’s largest coral reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2,300 kilometres over an area of approximately 344,400 square kilometres. The reef is located in the Coral Sea, off the coast of Queensland, Australia.
Wikitravel notes here: Day trips to the reef run from most of the length of the Queensland Coast. Generally speaking, the further north you leave from, the shorter the trip to the reef, as it approaches the coast near Cape Tribulation.
The general trip plan for the major operators provides for a early morning departure, Tie up a few hours later off a pontoon or cay where they have purchased rights, usually lunch is available (or included) and a late afternoon return to port. They always return to one or two specific locations, and it is not unusual for several boats to be within a few hundred metres of each other.
These types of trips are offered (at least) from the mainland coastal towns out of Cape Tribulation, Port Douglas, Cairns, Townsville, Airlie Beach (Shute Harbour), Mackay, Gladstone, and 1770 (from north to south). Check destination guides for more information, as not all trips run every day from every town.
For other resources about the Great Barrier Reef please see also these websites here: Greatbarrierreef.org, “Dive into the Great Barrier Reef” from National Geographic here and Great Barrier Reef Marine Park Authority here.
Attached Satelite image of the Great Barrier Reef from NASA: