If you're referring to the purple hovercar with guns: Ghosts are covenant lightly armoured (scout) vehicles seating only the driver. They are armed with twin plasma cannons and are used primarily for recon' or infantry support.
If you're referring to the freak occurrence of strange ghost-like players that jerk around, have no gamertag and behave erratically. It's a mystery really. They could be mods, remnants of test bots that Bungie haven't deleted properly or just weird things caused by lag.