Generator Of Random Numbers
Nowadays slot machines are computerized. So the odds are whatever they are programmed to be. In modern slot machines, the reels and lever exist only for historical and entertainment reasons. It is a Random Number Generator that chooses the reels positions. There is a Random Number Generator in the machine's software.
The RNG is constantly generating random numbers at an extremely high rate. As soon as the Play button is pressed, the most recent random number is used to determine the result. The result is different depending on exactly when the game is played. The result would be different a fraction of a second earlier or later.
Some professional gamblers observe that the RNG does not actually generate random numbers. Indeed, most RNGs will eventually repeat their number sequence. This behavior is due to poor programming. To build pseudo RNGs with very long periods is relatively easy. There is no computer that is able to complete a single period in the expected lifetime of the universe. Ronald Dale Harris, a former slot machine programmer, had access to the pseudo RNG code and seed values. It was he who discovered equations for specific gambling games like Keno. The fact that the next set of selected numbers would be based on the previous games played was revealed. But the RNG picks numbers even when the machine is not being played. So the player cannot see the secquence.

Payout Percentage
Slot machines usually pay out as winnings 82–98 per cent of the money that is wagered by players. The term theoretical payout percentage is used to denote it. Jurisdictions influence the minimum theoretical payout percentage. Law or regulation are used to establish it. A certain winning pattern is determined by every casino individually. It concerns the amounts they pay and the frequencies of these pay-outs. The selection of the slot machines winning patterns is chosen in such a way that it could yield a certain fraction of the money played to the house. The rest of the money goes to the players.
A slot machine's theoretical payout percentage is set at the factory when the software is written. Physical swap of the software is needed for changing the payout percentage after a slot machine has been placed on the gaming floor. It is done infrequently because this is a time-consuming process. There is sometimes a tamper-evident seal on the EPROM. It can only be changed in the presence of Gaming Control Board officials. According to other jurisdictions slot machines are randomly audited to ensure that they contain only approved software.
The technology being developed by the Nevada Gaming Commission would allow the casino's slot manager to change the game, the odds, and the payouts remotely. Only after the selected machine has been idle for at least four minutes can the change be done. After making the change the machine is locked to new players for four minutes, it displays an on-screen message informing potential players that a change is being made.

Machines That Are Linked
Offering of a particularly large prize, or jackpot becomes possible when a group of machines is linked in a special way. A small amount is contributed by each machine in the group to this progressive jackpot which is given to a player who gets a specific combination of symbols. A single slot machine's jackpot is considerably less than the progressive jackpot.
Sometimes multiple machines form multiple casinos. The machines are then owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss Programming
The reel display of modern slot machines is controlled by computer software. That is why the slot machine can be programmed to display combinations that are close to winning combinations.
This practice is called near-miss programming.
There is a related phenomenon that is also sometimes called near-miss. Winning percentages programmed into the slot machine control the chance of a winning combination appearing on a pay line. 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. The occurance of this phenomenon is possible when 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 Nevada Gaming Commission investigated the issue of a near-miss above or below the pay line. It was considered to be legal as long as the near-miss above or below the pay line was not specially programmed. In other words, any other combination must be just as likely to occur as the near-miss. It is impossible to program the machine to show winning combinations more frequently than other combinations above or below the pay line.
In Australia near-miss programming, where a near miss is inaccurately displayed is not allowed either. To audit manufacturer's practices regulators use stop motion cameras.

Fraud
Mechanical slot machines and their coin acceptors were sometimes considered to be cheating devices and other scams.
Modern slot machines are controlled by EPROM computer chips and coin acceptors were changed for bill acceptors. Different advanced anti-cheating and anti-counterfeiting measures have been taken to make it difficult to defraud these machines. Nowadays microwaves are used to defraud slot machines.

You must know exactly the game you are playing.
Always make sure that you insert the correct amount of coins into a machine. Before leaving a machine cash out your credits.
Only if you have fun your gambling won't be stressful. Use money management. Increase your bets when you are winning, and decrease your bets when you are losing.