On a regular slot, the jackpot is a fixed, known amount, whereas on a jackpot slot, the jackpot amount increases with each credit spent as you spin the reels on the jackpot game - with a tiny % of each credit being taken by the casino and added to the Jackpot prize-pool. The other key difference is that a progressive jackpot game will have an additional feature / screen which may take the form of a wheel of chance - where you have the opportunity to win one or more progressive jackpot prizes.