Most slot players are aware that they are not playing with a machine, but with a highly sophisticated computer. However, superstitious folks that slot jockeys are, some myths such as "machines about to hit" and "hot and cold runs" just refuse to die.
Most of us have heard of the "Random Number Generator" at the heart of the old-armed bandits evil brain, but few of us truly understand how it works.
I can't pretend to make a clear and simple explanation or something so arcane (that's the word of the day), but I will try to steer you in the right direction, should your inquiring mind want to know.
http://entertainment.howstuffworks.com/slot-machine3.htm This website is about as good as it gets.
In a nutshell, the RNG short for Random Number Generator, is a computer program that constantly spits out hundreds of random numbers per second.
The numbers correspond to stops on the slot reel through a mathematical equation (this is where the odds can be set), the higher value paying symbols and the chance of them matching up are programmed to hit less than the lesser paying values.
The numbers are assigned when you hit the button or pull the arm. This is important to know for two reasons -- 1. The chances of your numbers being the same as someone else's per pull is astronomical, meaning the player that just sat down at the machine you just left did not win or steal your jackpot. 2. Stopping the reels does not change the outcome of the spin.
The payback of the machine (between 75% and 99%) is set based on the lifetime of the machine (usually 3 to 5 years, although as technology advances, and machines live longer, that may be changing)
The real wildcard is how frequently the machine is played -- and how that is figured into the payback or lifetime quotient.
Noone has ever been able to explain that to me, but perhaps that is a trade secret.
The explanation of the RNG, also always claims that every pull has an equal chance of winning a jackpot. If that is true, I suspect those odds are severely stacked against the slot jockey.
Oh well, at least we can dream of one day riding a machine across the finish line and into the winner's circle.