The Sea of Japan is a protected part of the Pacific Ocean which is bordered by Japan on the east, Russia, China, North Korea, and South Korea on the west, the East China Sea to the south through the Korea Strait, the Sea of Okhotsk to the north and the Pacific Ocean through the straits between the islands of Japan. About 405,000 square miles or 1,048,950 square kilometers of ocean surface are enclosed in the sea. While it is shallow in the north and south which is best for the fishing there, areas of the sea are deeper than 10,000 feet or 3,050 meters. A warm current from Japan helps fishing in the north, changes the climate of the immediate region and helps keep Vladivostok the only ice-free port in eastern Russia.

An alternate name for the Sea of Japan is the East Sea which is a name promoted by South Korea, specifically VANK or the Voluntary Agency Network of Korea. To quote them,

"Using a proper name for the body of water between the Korean peninsula and the Japanese archipelago is not simply a question of changing the name of a geographical feature. It is rather a part of national effort by the Korean people to erase the legacy of their colonial past and to redress the unfairness that has resulted from it."

They have already succeed in convincing National Geographic and Lonely Planet Publication to use the designation of East Sea exclusively. More information about their campaign can be found here: http://www.prkorea.com/english/eastsea.html

Sources:
Rosenberg, Matt. Sea of Japan vs. East Sea About.com. January 11, 2004. <http://geography.about.com/library/weekly/aa022402a.htm>.
"Japan, Sea of." The Columbia Encyclopedia, 6th ed. New York: Columbia University Press, 2003. www.bartleby.com/65/. January 11, 2004.