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. Possibly very small rips, but no large rips or tears.
Good - The cover may have some writing, rips or tears. The pages may be yellowed significantly, dog eared, have a small amount of writing or small rips on them. But nothing that interferes with the ability to read the book.
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 readable. Pages may be significantly yellowed or have water damage or rips, but they are all readable. The spine is in tact,