Many people are surprised to learn that Green tea comes from the Camellia sinensis plant, just like black, Oolong, Darjeeling and white teas do. The difference is in the cultivation, processing and usually the cultivar. While technically you can make about any type of tea from any old tea plant, cultivars are carefully selected for best performance in the soil and climate where they are planted, so as to produce the best qualities for each type of tea.
There are myriad styles of green teas available on the market. I'm going to give you a brief description of a few of them here. Very brief. As in, this information is only a drop in the bucket when it comes to the subject of green tea.
Gyokuro Green tea is the finest grade of Japanese green tea. It is quite expensive, often costing 4x the price of sencha, or more. The tea plants are kept in shade for 20+ days before harvesting, and then processed like Sencha. The shade causes the plants to produce more chlorophyll, and results in a brighter green leaf, a more true green infusion, and higher caffeine content. The resulting tea is also slightly sweeter, has more Umami, and is lower in tannins. The procedure for steeping Gyokuro is different from other green teas. The water temperature is lower, and more tea leaves are used per cup. Gyokuro is grown in the Yame, Uiji and Asahina regions of Japan.
Matcha is typically Gyokuro that has been ground to a fine powder. This is the tea that is used for the traditional Japanese tea ceremony. I have also seen it used as an ingredient in baking recipes, as a seasoning and as an ingredient in smoothies. The caffeine content in matcha is also higher than Sencha (you are consuming the leaves, after all), and the flavor is much like Gyokuro.
Gunpowder is a Chinese style of green tea, produced by rolling the tea leaves into balls, rather than into cylinders. It is often pan-fired, which gives it a smokey flavor. It is called "gunpowder" because an early importer thought the rolled leaves resembled gunpowder.
Sencha is a Japanese style of green tea that is grown in full sun, and is harvested from the first flush (new leaves and buds from the earliest part of the growing season). Once the leaves are picked, they are steamed, rolled into cylinders, then dried and crumpled.
Sencha is typically produces a slightly bitter, slightly sweet infusion. It is the every-day tea of Japan. The tea is a light yellowish-green color, due to full sun the plants receive. Sencha is often used for flavored teas, because its own flavor is mild. This allows for a light, harmonious flavored tea, rather than strong flavors competing for attention.
Bancha style green tea is the second flush (harvest) of the same plants that produced the Sencha tea. It is considered a lower grade of green tea, and the flavor is stronger than Sencha.
Hyson green tea comes from the Anhui province of China. It is characterized by rolled leaves with a slightly twisted appearance. Though Hyson is considered by some to be a low grade of tea (sometimes it is, sometimes it isn't), it has a wonderful savory flavor.
A few final bits of green tea trivia for you:
Green teas are not oxidized before drying, but contrary to popular myth, green tea most certainly does contain caffeine.
Green teas are steeped at a lower temperature than black teas, around 175 degrees Farenheit, and for a shorter amount of time (about 2-3 minutes). You'll get a lot of bang for your buck with green tea, because you can re-infuse the same leaves 2-3 times.
I generally prefer to drink black tea and most herbals with a bit of sugar, but I don't feel the need for it with green tea. That's kind of handy if you're trying to cut down your sugar intake.