Playing the ghost is playing an imaginary opponent that, for the sake of the game, you assume will run out every time you miss. Thus, when playing the 'ghost', YOU must run out every time!!
The game is easy (in concept). It is played like this:
You rack the balls (9ball, 10ball, 12ball, etc.) and break.
Immediately after the break, you take ball in hand placing the cue ball anywhere you want on the table, and try to run out using standard rotation rules (lowest ball on the table must be the first ball contacted; combinations are allowed).
If you miss even one shot, you lose and 'the ghost' wins.