BitLotto was likely the first blockchain based "provably fair". It uses the transaction ID as the "ticket", and the results of a large government-run lottery to determine the winning numbers.
What does Provably Fair mean?
The terminology "Provably Fair" arose from the development of the Blockchain technology, during the rise of Bitcoin is based upon.
For the "gamer" it mean a revolution in such never-before-imagined things such as:
+ fair playing
+ guaranteed anti-cheating
+ instant payouts
+ public ledger
These gambling sites allow the public to see how outcomes are based on the gambler's input and a secret number that is disclosed and changed for the next rounds every hour for example.
This ensures the security to online gamblers to verify if the website's dice roll or card game.