Space-haulers are Terran starships used for transport. Active amongst the Fringe Worlds, many space-hauler captains reported mysterious lights and "foo-fighters," giving rise to the belief of alien species long before first contact was actually made.
References
Grubb, Jeff (February 27, 2001). StarCraft: Liberty's Crusade. Simon & Schuster (Pocket Star). ISBN 0-671-04148-7