NEW: Means new and possibly sealed, never played.
USED: Means second hand and one of the following..

VINYL

MINT: Never opened, still sealed or history known of the record to ensure it’s condition.
EXCELLENT/NEAR MINT: Opened and may have been played once or twice, no obvious wear on the record/sleeve.
EXCELLENT: Played but some signs of wear.
VERY GOOD: Obviously played and enjoyed with possible surface marks and/or wear to the sleeve.
GOOD: Gap fillers for collectors, with definite signs of wear on the record and the sleeve.

CDs/DVDs

MINT: Still sealed or unplayed with all other incidental items (if applicable) intact.
EXCELLENT: Second hand but with no obvious signs of play.
VERY GOOD: Played with a couple of  surface marks.
GOOD: Obvious surface marks but still plays, usually bought to fill gaps in ones collections before finding a better condition item.

Other objects are graded on a per item basis.