There are numerous sources available with in-depth information on tea - history, chemical makeup, production, agricultural practices. Below is a brief summary of information that you might find helpful.
Tea comes from an evergreen plant of the Camellia family (Thea Sinensis). The leaves of the Camellia sinensis plant are used for black, green, oolong, and white teas. The difference in taste and color are dependent on where it is grown, when it is picked, and how it is processed. Annual climate fluctuations, soil types, and packaging also affect taste.
Black teas undergo the most complex form of fermentation. The freshly plucked leaves are subjected to withering, rolling, fermentation, firing, and sifting/grading. Some smaller plantations still do some of this by hand; however, most of the larger plantations do a combination hand processing and the mechanized cut, tear, and curl (CTC) method. CTC teas are generally used for tea bags, whereas the larger leaves are generally sold as loose tea. The larger leaves produce a more flavorful tea. However, there are many who prefer teabags and better quality teabags can produce a very good tea. It is a personal choice.
Green teas are commonly referred to as "unfermented" teas. As soon as the tea leaves are picked, they are allowed to dry, then heat-treated to stop any fermentation. After the leaves are dry, they become a dull green. The last stage is sifting to separate the leaf size.
Oolong teas are commonly referred to as "semi-fermented" teas and mostly come from China. As soon as the tea leaves are picked, the leaves are wilted by the use of direct sun light and shaken in baskets to bruise the leaves. After the leaves are shaken, they are dried. This process is repeated until the leaves turn light yellow. The drying continues until the bruised leaf reacts with the oxygen (this reaction/oxidation phase is stopped after approximately 2 hours and the fermentation phase is about 20% complete. Formosa oolongs continue the oxidation phase a little longer.
White teas have become very popular within the last five years. New, unopened buds from the Camellia sinensis plant are picked for white tea. The buds are then withered and dried. After processing, the buds have a silvery/white appearance.
In order for tea leaves to release their flavors, hot or boiling water must be poured onto the leaves. The hot water allows the release of caffeine, polyphenols, and essential oils.
In general, black and oolong teas are best when the water has just come to a rolling boil, but by the time it is poured over the leaves the temperature is approximately 203 degrees Fahrenheit. White and green teas are better prepared with slightly cooler water. The ideal temperature for white and green teas is between 158 and 200 degrees Fahrenheit.