Slot machines are the most common around the world in casinos.  Why is that so?  Firstly, everything is automated and no croupier is required.  Secondly, it is a one man game; one gambler to one machine and privacy is ensured.  Thirdly, it is less space consuming.  But that is not the point.  The point now is how these machines work.  I have seen many PARS (Program and Reel Strips) sheets, which are blueprints of the machines.  Contrary to popular myth, the outcome of each round at the slot machine is determined by random numbers, and each play is completely independent.  Just because you win this time does not symbolify you will lose the next time purposely by machine.  Information of the reel stripping and probabilities of each win are sometimes printed on small cards and posted on the machines.  The following information is from a ‘Red, White and Blue’ machine.  The following is the pay table.  For new gamblers, 3 bar is in blue, 2 bar is in white and 1 bar is in red.  After the gambler slots in their bets, the spin button is pressed by him/her and the slot machine selects three random numbers, one for each reel.  These are chosen from a random number generator that generates random numbers at a rate of thousands per second.  The numbers are selected the moment the spin button is pressed.  When the bet has been placed but the button has not been pressed, the generators are already generating thousands of the numbers.  Once the button is pressed, the numbers are reached.  The payouts areas according: 

Pay Table For Red, White and Blue