These are quoted directly from abebooks.com, one of the largest used book resellers out there:
Condition of a book is usually in the form of VG/VG, Fine/Good, VG/--, etc. The first part is the condition of the book, the second is the condition of the dust jacket. If a "/--" is present, it usually means that the dustjacket is not present.
New - A new book is unread, in print and in perfect condition with no missing or damaged pages.
As New - To be used only when the book is in the same immaculate condition to which it was published. There can be no defects, no missing pages, no library stamps, etc., and the dustjacket (if it was issued with one) must be perfect, without any tears.
Fine (F or FN) - Approaches the condition of As New, but without being crisp. For the use of the term Fine, there must also be no defects, etc., and if the jacket has a small tear, or other defect, or looks worn, this should be noted.
Very Good (VG) - Describes a book that does show some small signs of wear - but no tears - on either binding or paper. Any defects must be noted.
Good (G) - Describes the average used worn book that has all pages or leaves present. Any defects must be noted.
Fair - Worn book that has complete text pages (including those with maps or plates) but may lack endpapers, half-title, etc. (which must be noted). Binding, jacket (if any), etc., may also be worn. All defects must be noted.
Poor - Describes a book that is sufficiently worn, to the point that its only merit is as a Reading Copy because it does have the complete text, which must be legible. Any missing maps or plates should still be noted. This copy may be soiled, scuffed, stained or spotted and may have loose joints, hinges, pages, etc.
It should be noted that many dealers will also use a "+" and "-" system to further explain the condition. For example, if a book is very clean, tight, bright, and crisp, but has a small imperfection, they will use VG+ instead of VG. We can't quite commit to calling it Near Fine, but Very Good doesn't quite do it justice.
Hope this helps in your next search.