Refurb - It was sent back to the manufacturer because their was a suspected or confirmed problem. The manufacturer has made any necessary repairs and it should be "good as new" - but I don't trust them.
Open Box - may be just fine - somebody purchased it and then returned it after opening. Maybe they bought a printer, opened the box, unpacked it, then realized it didn't have a parallel connection that they needed. Or perhaps the monitor they just bought just doesn't fit where the original buyer wanted it. Whatever the case, it was returned with no indication that there was anything wrong with it.
Used - used. Someone bought it, used it, then sold it. A la a used car.
