Mint (M) Perfect Condition basically means that it is as good as new, literally. The vinyl has to look new and sound as though it was never played, and the cover has to look untouched.
Near Mint (NM) Vinyl is near perfect. At most, a light scuff might appear on the record, but it plays without any noise or skips. The record sleeve should also be near perfect with only minor signs of wear. There is no "ring wear" on the cover.
Excellent (EX or VG++) Vinyl is playable with no skips of any kind. Some very minor surface scuffs might show, but they do not affect the sound quality. The record sleeve can have some ring wear but it should be mostly in good shape. Also, there can be some minor creases in the corners but no splits in the seams.
Very Good Plus (VG+) This is the grading category where the quality of the sound is affected -- there will be some surface noise -- and the vinyl will show wear, including surface scuffs and some light scratches. However, the record is still very listenable as there are no deep scratches that cause skips. On the cover, the corners may be slightly bent, but not broken. There may be wear to the seams but not tears or holes.
Very Good (VG) Vinyl doesn't necessarily look good, but still plays well enough. Surface noise will probably be present, including pops and clicks. At higher volume levels, the music should overpower any pops and clicks present. There may be seam splitting, but not complete separation of the sleeve panels from each other. There will be noticeable ring wear on the cover, but the cover should for the most part still be in good condition.
It is rare for me to post anything less than VG. Exceptions are sometimes made for rare collectors items.
Good (VG-) Vinyl is well played and looks it. However, the record should still play without skipping. It will have substantial surface noise, pops and clicks. Loss of various parts of the dynamic range will occur due to worn grooves. This grade depends a lot on the owner and the type of records he or she collects. A rock record may still sound ok at a high volume, but quieter styles of music will be practically unlistenable. The record cover may have many problems including seam splits, tears, and writing on it.
Fair/Poor (F/P) Using the word fair for this quality of record is, in the opinion of RCR, too generous and not recommended since it may be misleading. Records described as fair (F) or Poor (P) in the Goldmine grading system are ones that are basically unlistenable. Surface noise, skips and pops make hearing what's on the record nearly impossible, if not completely impossible. The cover is falling apart, torn, and has writing all over it.