Random Number Generator
There are many modern computerized slot machines. The odds are programmed. The reels and lever are present for historical and entertainment reasons only. It is a Random Number Generator that chooses the reels positions. The machine's software contains it.
The RNG generates random numbers at an extremely high rate. When you pres the Play button the result is determined by the most recent random number. The result varies depending on exactly when the game is played. A fraction of a second earlier or later, and the result would be different.
Some professional gamblers observe that the RNG does not actually generate random numbers. Most RNGs will repeat their number sequence. The cause of it is in poor programming. To build pseudo RNGs with very long periods is relatively easy. A single period can't be completed by any computer in the expected lifetime of the universe. Ronald Dale Harris, a former slot machine programmer, was the only one who had access to the pseudo RNG code and seed values. It was he who discovered equations for specific gambling games like Keno. Thanks to them the fact that the next set of selected numbers would be based on the previous games became known. But it is impossible for most machines, because numbers are picked by the RNG even when no one plays. So the player cannot tell where in the sequence they are.

Percentage Of The Payout
Slot machines are typically programmed to pay out as winnings 82–98 per cent of the money that is wagered by players. This is known as the theoretical payout percentage. The minimum theoretical payout percentage is controlled by jurisdictions. It is typically established by law or regulation. There is a certain winning pattern nearly in every casino. The amounts they pay and the frequencies of these pay-outs are reflected in it. The main criterion for selecting the winning patterns on slot machines is that they should yield a certain fraction of the money played to the house. As for the rest of the money it goes to the players.
A slot machine's theoretical payout percentage is set at the factory when the software is written. To change the payout percentage after a slot machine has been placed on the gaming floor a physical swap of the software is required. As it is a time-consuming process it is done infrequently. One can find a tamper-evident seal on the EPROM in certain jurisdictions. Only Gaming Control Board officials can change it. Other jurisdictions constantly ensure that slot machines contain only approved software.
The Nevada Gaming Commission is working now with Las Vegas casinos on technology that would allow the casino's slot manager to change the game, the odds, and the payouts remotely. The change cannot be done instantaneously, but only after the selected machine has been idle for at least four minutes. The machine must be locked to new players for four minutes and display an on-screen message informing potential players that a change is being made after the change is made.

Machines Linking
A group of machines offers a particularly large prize, or jackpot if they are linked together in a special way. Each slot machine in the group contributes a small amount to this progressive jackpot, awarded to a player who gets a specific combination of symbols. The amount of the progressive jackpot is far higher than any single slot machine's.
Multiple machines can be sometimes linked across multiple casinos. The machines are then owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss
The reel display of modern slot machines is under computer software's control. It is possible to make the slot machine frequently display combinations that are close to winning combinations.
Such showing combinations that are similar to winning combinations more frequently than would occur randomly is called near-miss programming.
A related phenomenon is sometimes called near-miss. The chance of a winning combination appearing on a pay line is controlled by the winning percentages programmed into the slot machine. The combinations above and below the payline are all roughly equally randomly distributed. There are roughly equally randomly distributed combinations above and below the pay line. This phenomenon only occurs where abbreviated physical reels are used to display a win pattern based upon the RNG. In video slot machines the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
The issue of a near-miss above or below the pay line was also investigated by the Nevada Gaming Commission. So long as the near-miss above or below the pay line was not specially programmed it was recognized legal. In other words, the near-miss must be just as likely to occur as any other combination. Winning combinations aren't supposed to be shown more frequently than other combinations above or below the pay line.
Australia also forbids to use near-miss programming, where a near miss is inaccurately displayed. Manufacturer's practices are audited by regulators with the help of stop motion cameras.

Slot Machines Fraud
Mechanical slot machines and their coin acceptors were sometimes susceptible to cheating devices and other scams.
Modern slot machines are controlled by EPROM computer chips and coin acceptors have become obsolete in favor of bill acceptors. It is difficult to defraud these machines because they their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures. Recent attempts at defrauding slot machines involve manipulating the EPROM, such as by directing microwaves toward it to disrupt its proper functioning.

I tis important to now what you are playing.
You should know exactly how many coins you are supposed to insert into a machine. Always cash out your credits before leaving a machine.
While playing just have a good time. Use money management. Increase your bets when you are winning, and decrease your bets when you are losing.