These same leaves formed the basis of most of pre-20th century trade between Europe and the orient. And despite the large variety of teas that now exist (Earl Grey, Darjeeling, Lapsang Souchong, Oolong, Green and even more exotically named ones like "Gunpowder"), they have one thing in common.
They all come from the same plant. Camellia sinensis. There exist different varietals of the plant, but yes, you read it right, it's all roughly the same. So why the differences in outcome?
Without going too much into the intricacies and chemistry of artisan tea, the short answer is in the process. Fermentation and drying, and in some cases smoking, causes the basic tea leaf to change in its properties and flavour. On one end of the spectrum, there's White and Green tea - only mildly dried before being packed and sealed off; on the other, there's Black tea: fermented, dried and sometimes smoked, these teas form the basis of what Europe is familiar with on its tea menus: English Breakfast, Earl Grey, Darjeeling etc. The different variations simply involve adding/blending in flavours, more commonly done with black teas rather than their less processed green cousins.
Suffice to say, the more treatment the leaves received, the less perishable they became, which explains the preponderance of black teas in Europe and America, while in Asia, particularly China and Japan, green teas are more commonly seen.
So do we head straight to the supermarket and pick up the biggest box of tea bags on the shelf? Not so fast - as it happens, most of commercially available teabags (especially the really cheap ones) contain the poorest quality of tea left from the fermentation and sifting process. While the whole leaf teas get tinned and sold to the big tea merchants (e.g. Mariage Frères, Dammann) or department stores (e.g. Harrods), the bits that fall through the sieves, along with the branches and stems of the tea plant, are what (mostly) go into cheap teabags.
The next time you pick up a teabag, just before you stick it in your glass, take a look at it. Are the leaves whole? Or are they powdery? If they're powdery, they will likely release more caffeine and tannins - great for a caffeine high (how about an espresso instead?), but not great if you're looking for the anti-oxidant, calming effects that everyone speaks of.
We've oversimplified, of course, but the fact is that with tea, there's certainly more than meets the eye.