Here are some commonly used abbreviations and terminology:

AU   -  About Uncirculated

BEP  -  Bureau of Engraving and Printing

Block  -  Prefix and suffix letter combination of serial number

Butterfly Fold  -  Fold along the corner of a note which after the cutting process results in an excess flag of paper, sometimes roughly resembling a butterfly's wing. By strictest definition this refers to gutter folds only, however some dealers and collectors also lump in unfolded exterior folds and make no distinction as long as an additional paper flag is present

CH  -  Choice

COPE-PAK  -  Currency Overprinting, Processing Equipment and Packaging. Machines employed by the Bureau of Engraving and Printing to first overprint currency, examine it for quality control, and then cut, band, and shrink-wrap the newly printed notes

CU  -  Crisp Uncirculated

Double Denomination  -  Error note featuring mismatched front and back values (eg. $10 face design with a $5 back design)

EF  -  Extremely Fine

F  -  Fine

First Printing  -  The back design, which is normally printed first

Foxing  -  Yellowish or brown staining. A form of environmental damage, not an error

Fractional Currency  -  Notes denominated in values less than a dollar ( eg. a Ten Cent note)

FRBN  -  Federal Reserve Bank Note

FRN  -  Federal Reserve Note

FW  -  Forth Worth, a branch printing plant of the Bureau of Engraving and Printing

G  -  Good

GC  -  Gold Certificate

Gutter Fold  -  Pleat-like fold(s) during one or more of the printings, which when unfolded results in a blank unprinted strip (aka "interior fold")

Misaligned Digit  -  Improperly turned character in serial number ( aka "turned digit")

Mismatched Serial Numbers  -  Note which has a different serial number on the left and right sides of the note. May be one or multiple numeral/letter/character different

Obstructed Printing  - Note missing a portion of a printing or printings because material, usually paper, falls between the printing plate and note

Offset  -  Wet ink transfer, normally in a mirror image, of a portion of one side's design onto the other side. This occurs when currency paper is not fed properly and ink ends up on the printing press cylinder instead of the intended currency sheet. The still wet ink on the cylinder is then transferred to the next sheet(s) to come in contact with it. Likely the most common misprint  encountered on modern U.S. currency

Overprint  -  Front serial numbers, seal(s), and on Federal Reserve Notes district numbers, and sometimes including other elements such as signatures and series designation on older issues, which are printed last ( aka "third printing")

PA  -  Pennsylvania

PCDA  -  Professional Currency Dealers Association

Plate Position  -  Letter and number combination, found on most U.S. currency, denoting where on a full sheet (typically column and row) a note was printed

Ragpicker  -  Colloquial term for a paper money collector

Red Seal  -  Nickname for United States Notes/Legal Tender issues because of their distinctive bright red seals

Replacement Note  -  Another term for "Star Note"

Rolled Digit  -  Improperly aligned serial number character which displays a portion of two numbers or letters

SC  -  Silver Certificate

Second Printing  -  The face design, excluding such elements as serial numbers, etc., which is printed after the back, and generally includes borders and portrait

Star Note  -  Modern note with a star symbol in the serial number distinguishing it as a replacement for a note that was removed because it contained a flaw that was caught. Please note, early U.S. currency, generally before Series 1899, often contain stars and other symbols as part of the normal serial number and do not represent replacement issues.

Stuck Digit  -  Another term for "Rolled Digit"

Third Printing  -  Another term for "Overprint"

Turned Digit  -  Misaligned character in serial number

USN  -  United States Note

VF  - Very Fine

VG  -  Very Good

Wet Printing  -  Method of printing currency on wet sheets of paper, widely used by the BEP prior to Series 1957, and which often results in minor size and design differences, along with a slight rippled texture, due to varying shrinkage between ink and paper drying

XF  -  Same as "EF",  Extremely Fine