The Time-Turner was a device capable of time travel. The Time-Turner resembled an hourglass on a necklace. The number of times one turns the hourglass corresponds to the number of hours one travels back in time. It is extremely important that the user of a Time-Turner not be seen by past or future versions of themselves unless, of course, said versions are aware of their usage of a Time-Turner. A possible scenario is a wizard/witch killing their past or future selves by mistake.