New Books - Looks like a new book you would pick up off of the shelf
Very Good - It's obviously been read. Pages may be starting to yellow, possibly some dog eared pages. The cover is mostly intact with minor scuffs or dings, but no large rips or tears.
Good - The cover may have rips or tears. The pages may be yellowed significantly or have many dog ears.
Acceptable - The book is readable but you're not buying it for the looks. May be missing the cover, but the spine and all the pages are in tact. Pages may be significantly yellowed or have slight water damage, but they are all readable. The spine is in tact,