In general, tea leaves can be classified as Green, Oolong or Black. The classification is a result of chemical changes which occur to the leaves when given a chance to oxidize, before drying, during the manufacturing process. Tea leaves that have been given full time to ferment become black tea. Oolong leaves are those that have been given a shorter time to oxidize or semi-ferment. Tea leaves which have been dried without being given time for oxidation or fermentation remain green in color.
Black tea leaves from India are graded based on various criteria. The most important determining factor is the size of the leaf which is not in itself an indicator of flavor or quality, though it can often influence the taste of the tea. Teas are then divided into broken grades and leaf grades. The broken grades are made up of smaller leaves and broken leaf particles. These teas usually produce stronger and darker teas. The smallest tea particles are labeled as dust and are most commonly found used in tea bags. Around 20 percent of teas produced are of leaf grades which are larger tea leaves. These rarer teas tend to produce teas which are smoother and lighter although not as strong as broken grades.
Here are tea grading "initials" which you usually only find on the side of chests and crates of loose tea.
OP - Orange Pekoe (it's pronounced peck-oh): This term is used to describe the largest grade of leaf for teas from Sri Lanka and occasionally from the South of India. The term Orange was derived from the Dutch house of Orange. Pekoe was derived from a Chinese word meaning white down and refers to the seemingly fuzzy tips of young tea buds' leaves.
FOP - Flowery Orange Pekoe: The term used throughout the rest of India to describe the largest tea leaves.
GFOP - Golden Flowery Orange Pekoe: FOP with golden tips which are the delicate yellow tips of the buds' leaves.
TGFOP - Tippy Golden Flowery Orange Pekoe: FOP with a larger proportion of golden tips than GFOP.
FTGFOP - Finest Tippy Golden Flowery Orange Pekoe: Very high quality large leaves.
SFTGFOP - Supreme Finest Tippy Golden Flowery Orange Pekoe: Very high quality large leaves with lots of golden tips. For Darjeeling teas, the "S" will denote a Supreme light color when brewed.
BOP - Broken Orange Pekoe: Broken tea leaves. You can find some good bargains with broken leaves, but as a general understanding broken leaves do not provide as nuanced a flavor as unbroken leaves.
BOPF - Broken Orange Pekoe Fannings: Tea fibers that are smaller than BOP leaves and are commonly found in tea bags. These do not provide as nuanced a flavor and aren't typically of very high quality.