For green tea, the tea leaves are harvested from the Camellia sinensis plant and are then quickly heated—by pan firing or steaming—and dried to prevent too much oxidation from occurring that would turn the green leaves brown and alter their fresh-picked flavor.
To put it simply...a tea is "only a true tea" if it actually contains tea plant leaves. And thus...this is why oolong, white, green and black are considered "true teas," as their leaves come from the actual tea plant camellia sinensis. By contrast, rooibos and herbal teas do NOT contain leaves from the tea plant.
Made from the leaves, buds and stems of the Camellia sinensis plant, oolong tea is slightly fermented and semi-oxidized, giving it a taste in between black and green tea. There are a wide variety of oolong teas, but the most famous oolong comes from the Fuijan province of China.
Unlike Chinese huángchá, Korean hwangcha is made similarly to oolong tea or lightly oxidized black tea, depending on who makes it – the key feature is a noticeable but otherwise relatively low level of oxidation which leaves the resulting tea liquor yellow in color.