Although all types of tea (Camellia sinensis) come from the same plant; they undergo different processes after they are picked to yield the different finished products.
White Tea: The most precious and rare type of tea; white tea can only be picked during early spring, while the buds are covered with whitish hairs. The leaves and buds are slowly dried, being only slightly oxidized to yield a naturally sweet and very mild flavor.
Green Tea: After being harvested, the tea leaves are put in the sunlight for a short period of time (usually 1-2 hours) to initiate oxidation. To halt the oxidative process (incorrectly referred to as fermentation by some) and to preserve the the freshness, the leaves are heated, before being rolled or flattened to help regulate the release of natural oils during steeping. Heating is usually done through either pan-firing or steaming the leaves before drying.
Oolong Tea: Think of bruising an apple or crushing mint leaves; after being picked, the leaves are repeatedly exposed to alternating sunlight and shaking in baskets to lightly bruise the edges. Some oolongs have more, some have less oxidation which is determined by how many cycles they undergo. At the desired point of oxidation, the leaves are pan-fired at extremely high temperatures to halt the oxidative process. The result is a dryer leaf than other types of tea, allowing for a longer shelf life and a malty and/or nutty flavor profile.
Black Tea: Freshly-picked leaves are placed in direct sunlight to wilt. This allows the leaves to be rolled without breaking or tearing the leaves. This releases chemicals that when exposed to oxygen and rested in humid rooms acts as a catalyst to oxidize the leaves. Like green and oolong, the black tea is heated to halt the oxidation, usually pan-fired or by baking. The result is a robust and full-bodied tea.
Tea was, and continues to be, consumed for medicinal and/or health benefits; this is largely because of various antioxidants that are naturally occurring in tea. However, there is an inverse relationship between the amount of antioxidants and caffeine. White and green tea have the highest level of antioxidants and the lowest amount of caffeine; while black teas have fewer antioxidants and more caffeine.