Ever wondered why a book in 'good' condition isn't really in good condition?  The mysteries of grading books explained! 

Fine - F

A Fine condition book should have no faults obvious to the eye. It won't
be perfect, it may have slight blemishes to the dust jacket or possibly
an inscription, but the binding should be tight and there should
certainly be no tears or markings on the dj or internally. It is very
common to call the book Near Fine (NF), a shortcut for booksellers who
don't want to list extremely tiny flaws.

Very Good - VG
This is the most common grade for a vintage book.  Many times it will be
qualified VG+ or VG- if there are slightly better or slightly worse
aspects of the book.  This might be called the average grade for a
non-current book. The book will show some wear and has clearly been
read. There may be some small tears or slight staining to the dust
jacket. Some wear to the extremities (corners and spine) is common. 
Flaws should not be major and scrupulous dealers will mention every
relevant detail. Book will be completely intact with no pages torn out
and nothing missing.

Good - G
When it comes to books, good
really isn't. This is one of the lowest grades possible and the book
might have a cracked binding, major tears to the dj, torn pages, a
missing map, and the like. Sometimes a book in good condition may be
termed a 'reading copy,' that is, a book that is not for collectors so
much as for the person who just wants to read it and is indifferent to
condition issues.

Poor - P
Pretty much the bottom of the barrel. Book might be in pieces, have major
water damage, torn up pages, crayon markings throughout, or other such
egregious faults.  Usually avoided unless extremely rare. Some
booksellers might also call this condition Fair, not really a good idea
because the F is already used by Fine.