What makes it green tea
Green tea is defined by prevention. Shortly after plucking, the leaf is heated enough to denature the enzymes that would otherwise brown it — steamed in Japan, pan-fired in most of China. Everything afterwards happens to a leaf that has stopped changing colour. This is why green tea is not 'unprocessed' tea, a description one meets often: it is heavily processed, just processed in a way that preserves rather than transforms.
A short history
Green tea is the oldest surviving form of made tea, and the reference point the other categories departed from. Powdered whisked tea was the dominant Chinese preparation during the Song dynasty and travelled to Japan with Buddhist monks, where it survives as matcha and chanoyu; China itself later moved to loose-leaf steeping and largely abandoned the powdered form. That divergence is why the same category is drunk in two such different ways in the two countries most associated with it.
How it is made, and the steam-versus-pan split
Pluck, wither briefly, fix with heat, roll or shape, dry. The fixation method is the fork in the road. Steaming preserves marine, umami and seaweed-adjacent notes and gives a vivid green liquor — this is sencha, gyokuro, matcha, bancha. Pan-firing in a hot wok adds toasted, chestnut and nutty character through direct contact with metal — this is Longjing, Bi Luo Chun, Huangshan Maofeng, and also Korean green tea. Shading the bushes for a week or more before plucking, as with gyokuro, kabusecha and matcha, pushes the savoury character further still.
What it tastes like
There is no single green-tea flavour, which is the main thing to take away. Japanese steamed teas run marine, savoury and grassy, with gyokuro so thick and sweet it barely reads as green tea. Chinese pan-fired teas run nutty, chestnut-sweet and clean. Roasting a Japanese green tea produces hojicha, which is toasty and caramel-like and has almost nothing in common with the sencha it started as. Anyone who dislikes green tea has usually met one branch of this family and not the other.
Brewing, and the mistake nearly everyone makes
Green tea is the category most often ruined by water that is too hot. Catechins, which carry bitterness and astringency, extract far more readily at high temperature than the amino acids that carry sweetness and savoury depth — so boiling water extracts the harsh compounds efficiently and the pleasant ones poorly, producing a cup that is thin and bitter at once. Most greens want 70 to 85 °C; gyokuro wants 50 to 60 °C with several times the usual leaf. The exceptions run the other way: hojicha and bancha take near-boiling water happily.
Common misconceptions
That green tea is low in caffeine: matcha in particular is not, because the whole leaf is consumed rather than strained away. That it is unprocessed. That 'green tea' names a flavour, when it names a process. And that bitterness means the tea was bad — far more often it means the water was too hot, which is the one variable most drinkers never think to change.