The purpose of grading tea leaves is not only to create teas that have a uniform leaf density and appearance, but more importantly to separate larger from smaller particles of leaf since they will extract at different rates when brewed.
Grading is often assumed to mean grading for quality, but quality is determined by the taste of the tea, not its appearance. However, the leaf shape and size of the finished tea has much to do with the characteristic flavor of the resulting cup. This is because leaves rolled in a given way adopt a particular flavor that differs from the same leaves rolled in a different fashion. It can also be safely said that the more broken the leaf is, the faster brewing the cup, and typically the more body and pungency the tea will have.
The term "orange pekoe" is often taken to mean a type of tea. Actually, it is a tea industry term that refers only to specific leaf size. Here are some common leaf grades for black teas:
WHOLE LEAF
S./Souchong
A bold, twisting leaf, often light in liquor. China is the most common producer of this grade.
F.O.P. - Flowery Orange Pekoe
A long leaf with a slightly open, "crushed flower" appearance. India is the most common producer of this grade.
O.P. - Orange Pekoe
A thin, wiry leaf with a tighter roll than F.O.P. T. and G. - Tippy and Golden Modifiers that are liberally used with both whole leaf and broken grades to indicate the presence of colorful tips in the dry leaf.
BROKEN LEAF
P. - Pekoe
A curly, large broken grade, typically without visible tip. Sri Lanka is the largest producer of Pekoe grade.
B.O.P. - Broken Orange Pekoe
Small, squarish grade with good body and strength. India produces the best B.O.P.s.
FANNINGS & DUST
F. - Fannings
Smaller than B.O.P., with less keeping quality, and used for commercial tea bags. The name derives from traditional practice in which the broken grades were tossed in front of a fan, and the small particles blown off were called the fannings.
D. - Dust
Smallest grade produced; quick liquoring.