Pachinko is a mixture between slot machine and pinball. The player is quite passive while playing pachinko. He or she is only controlling the speed with which many small steel balls are thrown into the pachinko machine.
Most of the balls just fall down the machine and disappear, but a few find their way into special holes. This activates a kind of slot machine. As in the slot machine, you win if the same three pictures appear. This occurs quite rarely in pachinko, but if it happens, you win countless new balls. When just playing for 500 or 1000 Yen, you may likely just lose all your balls within a few minutes.