UNC = 'Uncirculated' - Perfectly preserved, sharp corners, flat, mint condition. As though it were fresh from the printers.
XF = 'Extremely fine' or 'EF'. Only slightest evidence of use - a crease, up to three light bends or minor marks etc.
VF = 'Very Fine' - Several creases, slight edge+corner wear etc., but still clean and fairly crisp, with no tears.
F = 'Fine' More worn and dull, but no centre hole from excessive folding. Could have edge tears, fading, pinholes etc.
VG = 'Very Good' - Tears into the design, staining, small center hole from excessive folding, graffiti. No pieces missing.
G = 'Good' - Usually limp, with heavy soiling and/or small pieces missing etc.