The global ocean system has five main divisions: the Pacific, the Atlantic, the Indian, the Arctic and the Southern oceans. Each area is loosely defined by the dimensions of its basins and the continental landmasses that surround them. In places where no clear delimitation exists, the boundaries between oceans is arbitrary.
The Pacific Ocean is the largest and deepest division of the ocean system. It covers an area of 155 million square kilometers and contains more than half of the world's water. The Atlantic is the world's second largest ocean. It's also the youngest, having opened up only 150 million years ago during the Jurassic Period. The Indian Ocean is what remains of the ancient Sea of Tethys. This ocean is arbitrarily bounded on its western side by a line stretching from South Africa's Cape Agulhas to Antarctica.
All three of these oceans are delineated by a line drawn at 60 degrees south latitude. Water located south of this line is in the Southern Ocean. The Southern Ocean rings Antarctica and grazes the southern tip of South America. The Arctic Ocean is the smallest and shallowest of the five divisions. It's nearly landlocked, and its interior is permanently covered with thick sea ice.