Black tea, green tea, Oolong tea and white tea all come from the same plant.
The difference is in the process of preparation.
Black tea is oxidized and fermented. Black tea has a bitter taste.
Green tea is not fermented but it is steamed and has a more grassy flavor.
Oolong is only partially fermented.
With White tea the leaves are picked only twice a year at a very young stage just before the buds are beginning to open.
The leaves are then partially fermented in a similar manner to the Oolong tea.
Some Links to Dig Deeper...