A progressive jackpot is a jackpot (a gambling grand prize or payout) which increases each time the game is played but the jackpot is not won. When the progressive jackpot is won, the jackpot for the next play is reset to a predetermined value, and resumes increasing under the same rule.
Progressive slots are the best ones online if you're looking for huge prizes. They can also be very volatile, but if you're up for it and want the chance at the biggest wins, then progressive slots are your thing. Regular ones do not offer such big prizes, but they are more consistent and provide smaller wins.
