A known shipper is a person or business that is in the Known Shipper database maintained by the Transportation and Safety Administration (TSA). An known shipper can submit cargo to be shipped on an aircraft that carries passengers so long as that cargo has been screened and is not "personal effects".
Personal effects are always considered "unknown cargo" even when the shipper is known, and can only travel by cargo aircraft unless they are travelling on the same passenger aircraft flight as the shipper.
Known cargo is cargo by a known shipper of a commercial nature containing no personally used items.
Known cargo can be used goods, but must be packed and shipped by a known shipper.