Volume in perfect condition that is currently available from the publishers.
An out of print book that looks as new, maybe with an old price label on the inside of the dust jacket but otherwise could be given as a gift.
Light signs of age or wear, maybe very slight fading of the cover or discolouration of the edge of the pages, dust jacket possibly with light indentations or signs of wear at the edges - maybe sticky mark residue where a price label has been - good enough for the shelf of most of us but maybe not quite good enough to pass as new if you were giving it as a present.
A book that has evidence of having been read but with no significant creasing, writing, folded down corners, marks or damage. Essentially this will be a well looked after book that has the normal signs of wear you would expect from a good book that has been taken from the shelf and read a number of times.
A book that has been very well used. There will be no major damage such as pages missing or torn, however there may be turned down corners, marks, worn/faded dust jacket or well thumbed pages. This sort of book would be of interest only if it was particularly rare or was required for the reference content within and condition was not important.
Sometimes you may see a book given two ratings such as Good/Excellent. In this case the first rating will refer to the cover or dust jacket, the second to the body of the book.