A: Orthodox Leaf Grades
(1) OP - Orange Pekoe Despite popular misconceptions, OP has nothing to do with orange the fruit or orange the colour. It does not even denote a place of origin, but rather a specific leaf--the third one on the plant. OP leaves are left whole or in large pieces during processing, and generally have a dark, well twisted appearance.
Historical note: "Orange" is said to refer to the Dutch House of Orange; "pekoe" is a corruption of the chinese word for "white hair" and refers to the fine downy growth on the underside of the leaf.
(2) FOP - Flowery Orange Pekoe refers to teas produced using the top two leaves of the new growth, and gives a more flowery result than OP. Since the leaf yield of these small leaves is less than for OP, the result is scarcer and commands higher prices at auction.
(3) GFOP - Golden Flowery Orange Pekoe is as FOP, but containing a high proportion of "golden tips" which refer to the unopened bud at the top of the plant. In general, the colour of the dry leaf is brighter, and the tips should be very noticeable. For a tea to contain these tips means that it was picked quite young, when the buds were still tender and fresh.
(4) TGFOP - Tippy Golden Flowery Orange Pekoe has more of the sweet flavoursome tips than GFOP, and is an extremely high grade.
(5) FTGFOP - Fine Tippy Golden Flowery Orange Pekoe is generally accepted as the most "tippy" of the leaf grades, and is the most sought after, especially for Darjeeling teas.
(6) SFTGFOP - Super Fine Tippy Golden Flowery Orange Pekoe An extremely rare find, this grade supercedes (5) above. For Assam teas, the leaves contain sweet golden tips in abundance, and are consequently referred to as "GT."
Note: The suffix "1" is sometimes added to the above grades (5) & (6) to denote an even higher grade. Thus, the theoretically finest leaf would be SFTGFOP1!
B: Orthodox Broken Grades
(1) BOP - Broken Orange Pekoe is, as its name suggests, an OP where the leaves are cut into fine pieces, allowing more oxidisation and a stronger flavour and colour. The pieces are larger than CTC or dust, and a strainer should be sufficient to keep them out of the cup. For Ceylon teas, BOP is the most sought after grade.
(2) BOPF - Broken Orange Pekoe Fannings are the smaller pieces which are left behind when the BOP is taken out. Due to their increased surface area, they produce a very well coloured and strong tea, but will fall through a strainer. Thus, they are usually found in tea bag form.
(3) D - Dust is literally the sweepings from the factory floor. Most of it is reputed to end up in tea bags...