Black tea is fully oxidized. That is, the oxidation process is allowed to stop on its own. After oxidation, the tea is dried. It is oxidation that makes the tea leaves dark. Black tea has a strong, often earthy taste and is the most common type drunk in the Western world.
Most people prefer to drink black tea with milk, but high quality teas such as Assam, Keemun and Darjeeling are also lovely on their own.