I don't know alot about them, I don't think they were ever explained very well. Here is what the HP Lexicon says about it... I really thought they would have more info then this....
Time-Turner The Time-Turner is a small silver hourglass worn on a chain around the neck. It's a very powerful and dangerous magical item which literally turns back time for the user, one hour per inversion of the glass.
Hermione used a Time-Turner for the 1992 - 1993 [Y12 - Y13] school year to allow her to attend more classes than there were hours for in the day (PA21).
Hermione and Harry used a Time-Turner to go back in time three hours and rescue Buckbeak and Sirius Black from a terrible fate (PA21).
Time-Turners are carefully controlled by the Ministry of Magic (PA21).
From the lexicon...
Here is what the HP Lexicon says about it... I really thought they would have more info then this....
http://www.hp-lexicon.org/magic/devices/devices-t.html#time-turner
Time-Turner
The Time-Turner is a small silver hourglass worn on a chain around the neck. It's a very powerful and dangerous magical item which literally turns back time for the user, one hour per inversion of the glass.
Hermione used a Time-Turner for the 1992 - 1993 [Y12 - Y13] school year to allow her to attend more classes than there were hours for in the day (PA21).
Hermione and Harry used a Time-Turner to go back in time three hours and rescue Buckbeak and Sirius Black from a terrible fate (PA21).
Time-Turners are carefully controlled by the Ministry of Magic (PA21).
See also TIMEPIECES.
Yep.. so helpful. LOL