There are many slot machines in the world but they basically break down to two kinds of slot machine games, I guess three if you throw in the ones that don't ever pay or that seldom pay. I'm sure they exist and likely in larger numbers and locations than would please most of slot gamblers.
The first of the two slot machine games is the first kind that came out which involved actual reels that spun and if they stopped in a winning position they unlocked a spring which dropped out the jackpot. This would be what you would call an all or nothing slot machine and it was eventually upgraded to have different sized payouts depending upon where the reels stopped.
This was all great except that the machines weren't providing enough excitement to keep some of the players effectively entertained so they'd want to spend longer periods of time gambling. Eventually came the second type of slot machine game some time after they changed over from having actual reels inside them to having computer chips using a computer program first created for video poker games which is called a "random number generator".
This new computer program made many new things possible in the world of slot machines because where there once was a limited number of possible winning positions for the slot reels to land, ... now existed literally an infinite number of ways to get money back to the players. These possibilities gave slot machine creators nearly as many avenues in which to take the future of slot machine play. The first problem was obvious and easily eliminated now that the slot machine games had become computerized. There weren't enough wins to keep some of the players occupied happily over long stretches of time. They had already tried setting the old "reel" slot machines so that certain combinations paid back often, but because of the limited number of payout slots in the reels, only a limited number of small wins could be attributed to any one machine. That too, was no longer an obstacle. So they merely created many ways to win small amount pay-offs and the win combined with the variety of ways it happens was enough to satisfy the problem at hand.
The need to ever-increase the ways of keeping players happy has been recognized with new vigor and online casinos are leading the way in that aspect. Not only are they inventing new and exciting ways to get money back to the players but they have also added brand name themes to slot machine play taking following the lead set by the "Wheel of Fortune" slot that was introduced to land casinos some years ago. Now you can expect to find themes such as the ever-popular "Laura Croft: Tomb Raider" which now has released a second version said to re-invent slot machine play.