The majority of tea is of the black variety where the tea leaf has been fully oxidized and fermented. Popular black teas include Orange Pekoe and Darjeeling. Darjeeling teas can also be further categorized into summer and spring varieties, depending on how soon young tea leaves are picked. Green tea is not oxidized and has a less hearty flavor than black tea. Green tea has been a favorite in Asia for many years, and is just now becoming more popular in the States.
Oolong tea is a hybrid between green and black teas.
Surprisingly enough herbal teas are not true teas at all. Herbal teas are primarily made from roots, berries, and other plant materials. A testament to the fact that tea is no longer a simple manner of brewing a leaf is the complex tea grading system established over the years. Tea grading is not standardized worldwide and many countries have developed their own yardsticks by which to grade their teas. According to Joy Edlund from Stash Teas, black tea grading in general reflects how the leaf was picked and how it holds up after the oxidizing process.