All tea comes from the same plant (camellia sinensis).
Black tea and oolong tea are fermented to convert caffine analogs into caffine molecules. This process also darkens the leaves and changes the flavor profile of the tea.
Green tea and "White" tea are both steam processed as soon as possible after picking, preserving the tea leaves in as natural a state as possible. This prevents breakdown of the polyphenolic compounds that act as anti-oxidants.
The difference between White tea and Green tea is when the leaves are actually picked. White tea is picked very early in the maturation process of the tea leaves, often while the leaves are still budding. The leaves themselves are usually covered in plant hairs and have a whiteish appearance.
In China, white tea is considered very healthy for women.