Latitudes between 40 and 50 degrees south of the equator. Named thus by seamen because the lack of any significant landmass in those latitudes allows for an unencumbered zone of strong, steady westerly winds prevalent throughout the year.
The stormy conditions produced in the sea result in waters rich in oxygen and nutrients in which phytoplankton thrives and provides abundant food for vital lower parts of the food chain such as krill. Although desolate and inhospitable by human standards, this part of the world is rich in marine life and highly biodiverse.
The roaring forties may be a reason for the extreme cold and ice formation in Antarctica, providing thermal insulation from the warmer air of temperate latitudes further north.
Sources:
Columbia Encyclopaedia
1997 Dutch Whitbread Race team