Great Pacific Garbage Patch

The Great Pacific Garbage Patch, also described as the Pacific Trash Vortex, is gyre of marine litter in the central North Pacific Ocean located roughly between135°W to 155°W and 35°N to 42°N. The patch extends over an indeterminate area, with estimates ranging very widely depending on the degree of plastic concentration used to define the affected area.

The Patch is characterized by exceptionally high concentrations of plastics,chemical sludge and other debris that have been trapped by the currents of the North Pacific Gyre in a relatively stationary region of the North Pacific Ocean (commonly referred to as horse latitudes). The patch consists primarily of suspended particulates in the upper water column (and is therefore not visible from satellites). 

The Great Pacific Garbage Patch formed gradually as a result of marine pollution gathered by oceanic currents.The gyre's rotational pattern draws in waste material from across the North Pacific Ocean, including coastal waters off North America and Japan. As material is captured in the currents, wind-driven surface currents gradually move floating debris toward the center, trapping it in the region.

It was recently in the news as the debris caused by the Japan Tsunami of 2011 is said to have floated towards this garbage patch.

A similar patch of floating plastic debris is found in the Atlantic Ocean. See: North Atlantic Garbage Patch

A gyre is any large system of rotating ocean currents. The 5 major gyres are: North Pacific Gyre, South Pacific Gyre, Indian Ocean Gyre, North Atlantic Gyre and South Atlantic Gyre.

Sources: 

http://en.wikipedia.org/wiki/Great_Pacific_Garbage_Patch

NCERT Fundamentals of Physical Geography. Class 11.