Tea comes from a species of Camellia, known as Camellia Sinensis, or Chinese Camellia. There are two major varities of this plant: Camellia sinensis sinensis and Camellia sinensis assamica, or the Chinese and Assam (a region in India) varieties. The Assam variety is actually a hybrid of the original Chinese variety and a native tea plant.
Generally, all teas come from this plant. The differences between the different types of tea, including green, black and oolong teas, is in the processing of the leaves, and in variations of the land in which the plant is cultivated. Black teas go through a process of oxidisation (fermentation), oolong less so, and green teas are not oxidised at all.
Camellia Sinensis Sinensis
This is a small leafed plant, and is used largely in the Chinese and Japanese green teas. However, it can also be used to make black teas, such as the English Breakfast Tea
Camellia Sinensis Assamica
This has a larger leaf and is used largely in the black teas. However it is also blended to some extent with the Sinensis variety, and there are also green and oolong teas from this variety.