The Great Pacific Garbage Patch is an area of debris in the North Pacific Gyre.
The center of the North Pacific Gyre is relatively stationary region of the Pacific Ocean (the area it occupies is often referred to as the horse latitudes) and the circular rotation around it draws waste material in. This has led to the accumulation of flotsam and other debris in huge floating 'clouds' of waste which have taken on informal names, the Great Pacific Garbage Patch, the Plastic soup, the Eastern Garbage Patch or the Pacific Trash Vortex. While historically this debris has biodegraded, the gyre is now accumulating vast quantities of plastic and marine debris. Rather than biodegrading, plastic photodegrades, disintegrating in the ocean into smaller and smaller pieces. These pieces, still polymers, eventually become individual molecules, which are still not easily digested. Some plastics photodegrade into other pollutants. The floating particles also resemble zooplankton, which can lead to them being consumed by jellyfish, thus entering the ocean food chain. In samples taken from the gyre in 2001, the mass of plastic exceeded that of zooplankton (the dominant animal life in the area) by a factor of six. Many of these long-lasting pieces end up in the stomachs of marine birds and animals.
For several years ocean researcher Charles Moore has been investigating a concentration of floating plastic debris in the North Pacific Gyre. He has reported concentrations of plastics on the order of 3,340,000 pieces/km² with a mean mass of 5.1kg/km² collected using a manta trawl with a rectangular opening of 0.9x0.15m² at the surface. Trawls at depths of 10m found less than half, consisting primarily of monofilament line fouled with diatoms and other plankton.
The size estimate of the patch varies depending on the source, with some claiming that it's twice as large as the continental United States. Researcher Dr Marcus Eriksen believes the Great Pacific Garbage Patch is in fact two massive areas of swirling rubbish that are linked. Eriksen says the gyre stretches from about 500 nautical miles off the coast of California, across the Northern Pacific to near the coast of Japan.
Occasionally, shifts in the ocean currents release flotsam lost from cargo ships into the currents around the North Pacific Gyre, leading to predictable patterns of garbage washing up on the shores around the outskirts of the gyre. The most famous was the loss of approximately 80,000 Nike sneakers and boots from the ship Hansa Carrier in 1990: the currents of the gyre distributed the shoes around the shores of British Columbia, Washington, Oregon, and Hawaii over the following three years. Similar cargo spills have involved 29,000-30,000 plastic yellow ducks, blue turtles and green frogs bathtub toys in 1992 and hockey equipment in 1994. These events have become a major source of data on global-scale ocean currents. Various institutions have asked the public to report the landfall locations of the objects (trainers, rubber ducks, etc.) that wash up as a method of tracking surface waters' response to the deeper ocean currents. 
The gyre is discussed in Alan Weisman's The World Without Us as an example of the near-indestructibility of discarded plastic.
[ Source: Wikipedia ]