At night, police cars will sometimes be parked in 'hidden areas', such as behind a billboard or in a parking lot, and will be facing traffic looking for speeders or DWI cases. They may also be parked alongside the road or in the breakdown lane on the highway.
Ghost cars will always have their head lights off, but sometimes the day-running lights will remain on, giving the headlights a light 'ghostly' appearance. It is also sometimes used to refer to any ordinary car driving at night with no headlights or just their dayrunners on, but usually the title is reserved for police vehicles.
When driving down a country road at night, or anywhere without street lights, flick off the car headlights for a second. Looks cool when driving also freaks out anybody who happens to be around when you do it.