New: Straight from the factory. These items are either new boxes that have never been opened or are bricks that have been parted out from sets and have never been played with.
Does that mean the bricks are in perfect condition? Well, sort of. Bricks come pre-packaged by LEGO and bounce and roll around in their box meaning that they can rub against each other and pick up small scratches and imperfections. We can guarantee they've never been connected with another piece.
New, Sealed: The seals from LEGO are still intact and have never been opened.
New, Complete: The set is whole, the bags are still sealed, but for some reason the box has been opened. Any additional conditions will be mentioned in the description.
Used: Exactly as it says. No reason to throw away perfectly good bricks just because someone else played with them. One of our missions is to save as much plastic from the landfills as possible. Used will never be in perfect condition, but it will be in played condition. If we wouldn't build with it, we won't sell it.
Used, Complete: Typically applies to sets. All parts needed to complete the set as described in the manual are present and verified. This does not mean that the box is included, stickers will typically have been applied to parts, minifigures are complete and in good condition unless otherwise noted.
Used, Incomplete: Something is missing and will be noted in the description.