Brand New - New, unread, and unused.
Like New - Unused, no missing pages, all pages undamaged, and no underlining/highlighting. Slight shelf wear on the cover.
Very Good - Doesn't look brand new but has no easily noticeable damage to the cover, no missing pages, all pages undamaged, no underlining/highlighting, no writing in margins, very minimal identifying marks inside cover, very minimal wear and tear. You would give it to a friend as a gift.
Good - Cover has some wear, mostly corners and binding. Majority of pages undamaged, minimal pencil underlining of text, no missing pages. You would use it yourself, but wouldn't necessarily give it as a gift.
Acceptable - Cover has wear mostly corners and binding. Possible writing in margins, possible underlining and highlighting of text, no missing pages. Book beaten up a bit but it works.
This is how we try to rate our books. However, be sure to read each description carefully. Some books may have special cases. If you have any questions about this, please ask.