Ghosting is where you are essentially removed from the view of other players and ATC. you are able to complete your flight, but nobody else is able to see or interact with you. To add to this, you loose all of your XP and receive 7 violations I believe.
Ghosting on the training server will typically only happen by the system; this can be for acquiring too many violations in a single flight.
On the expert server, IFATC and some select individuals can ghost you for trolling, neglecting ATC, and the same scenario as training server.
They take a week to wear off, and typically are not removed. If you receive too many, they will prevent you from accessing the expert server.