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 exactly does Provably Fair mean?
The terminology Provably Fair arose from the development of Blockchain technology, of which Bitcoin is based upon.
For the "gamer" it mean a revolution in such never-before-imagined things such as: a) fair playing, b) accountability, c) guarentee anti-cheating, d) instant payouts, e) public ledger, etc.
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.