I would highly recommend viewing the Wikipedia page on Tea. It has a lot of information about so many different types of tea and each process. The four main teas are black, oolong, white, and green.
Black is considered the most "western" of the teas as many people in the U.S. and Britain enjoy it the most. The other three are becoming more and more well known as the years go by and I would not consider them to be solely "eastern" teas anymore. These four teas come from theCamellia sinensisplant.White tea, essentially, is where the leaves are picked and dried quickly to prevent oxidation to occur. Green tea is minimally oxidized and black is fully oxidized. Oolong is somewhere in between green and black tea.