An A+ record is an organically conceived masterpiece that repays prolonged listening with new excitement and insight. It is unlikely to be marred by more than one merely ordinary cut.
An A is a great record both of whose sides offer enduring pleasure and surprise. You should own it.
An A- is a very good record. If one of its sides doesn't provide intense and consistent satisfaction, then both include several cuts that do.
A B+ is a good record, at least one of whose sides can be played with lasting interest and the other of which includes at least one enjoyable cut.
A B is an admirable effort that aficionados of the style or artist will probably find quite listenable.
A B- is a competent or mildly interesting record that will usually feature at least three worthwhile cuts.
A C+ is a not disreputable performance, most likely a failed experiment or a pleasant piece of hackwork.
A C is a record of clear professionalism or barely discernible inspiration, but not both.
A C- is a regrettably successful exploitation or a basically honest but quite incompetent stab at something more.
A D+ is an appalling piece of pimpwork or a thoroughly botched token of sincerity.
It is impossible to understand why anyone would buy a D record.
It is impossible to understand why anyone would release a D- record.
It is impossible to understand why anyone would cut an E+ record.
E records are frequently cited as proof that there is no God.
An E- record is an organically conceived masterpiece that repays repeated listening with a sense of horror in the face of the void. It is unlikely to be marred by one listenable cut.
Christgau's Record Guide: Rock Albums of the '70s, 1980