Widely thought to be one of the most dangerous areas of ocean in the world, Cape Horn is the area at which the great oceans of the Pacific and the Atlantic meet. This is found south of the Wollaston Islands (the group of islands that create the treacherous Strait of Magellan) at the southernmost point of Chile in South America. The ocean floor begins to shelve upwards at this point, creating quite shallow waters (for the ocean!). This leads to short and choppy seas. It is also subject to violent gales originating in Antarctica and the combination of a rising sea floor, high winds and immense currents as the water pours from the Atlantic to the Pacific and back creates vast waves many meters high.
Cape Horn was "discovered" in January 1616 by Willem Corneliszoon Schouten who was searching for an alternative route to the Pacific instead of braving the Strait of Magellan. The Magellan Straits are narrow, treacherous, and until recently considered to be populated with "unfriendly" (or curious!) natives. The new passage did not prove to be as popular as was hoped due to the adverse sea conditions, and horrific weather which plagues the cape for two thirds of the year. To pass too close to land puts ships at the mercy of the fierce currents, to travel too far south means the threat of icebergs. It's a harsh choice! The mythos surrounding navigating the Cape is possibly why one of the more popular brands of auto helm for yachts is known as Cape Horn.
Good accounts of rounding the Horn can be found in Joshua Slocum's Sailing Alone Around the World, Pete Goss' Close to the Wind and Webb Chiles' Stories of Storms and Survival. Also, see http://www.nautica.it/charter/capehorn.htm for a more detailed account of the cape and it's history.