Slot machines' technology has experimented many changes over the years. At the beginning they had a mechanical design, and nowadays that has been almost completely computerized. Although, the game is still basically the same.

The reel slot machine has a central metal shaft connected to a handle mechanism that holds the reels. That handle mechanism is the one that gives things movement. A braking system is needed to stop the reels after the spin, and also sensors to detect the reels position so they can tell the payout. Of course, that a coin detector is essential to detect that a coin has been inserted and that also unlocks the handle so it can be moved.

Conventional and electrical slot machines, work on similar principles. The big differences are that in an electrical machine, the spinning happens by a motor and the breakers that stop the spin are activated by solenoids. And of course, that they have more advanced money-handling systems.
It is a common belief that the odds on a machine have something to do with the number of each kind of symbol on each reel, but this is not the case. Modern slot machines are computerized, so that the odds are whatever they are programmed to be. For instance, if the jackpot combination is "7-7-7", slot machine owners can fool/tease people by making "7-7-(non-7)" come up frequently. In modern slot machines, the reels and lever are present for historical and entertainment reasons only. The positions the reels will come to rest on are chosen by a computer chip as soon as the lever is pulled or the "Play" button is pressed.
Slot machines are typically programmed to pay out around 82-98% of the money that goes into them as winnings. The winning patterns on slot machines, the amounts they pay, and the frequency at which they appear are carefully selected to yield a certain percentage of the cost of play to the "house" (the operator of the slot machine), while returning the rest to the player during play. Suppose that a certain slot machine costs $1 per spin. It can be calculated that over a sufficiently long period, such as 1,000,000 spins, that the machine will return an average of $950,000 to its players, who have inserted $1,000,000 during that time. In this (simplified) example, the slot machine is said to pay out 95%. The operator keeps the remaining $50,000. The payout percentages are set at the factory when the casino orders the machines. Changing the payout percentages after a slot machine has been placed on the gaming floor requires a physical swap of the game software which is usually stored on an EPROM but may be downloaded to Non-Volatile Random Access Memory (NVRAM) or even stored on CDROM or DVD depending on the technological capabilities of the machine and the regulations in each jurisdiction. In Nevada, the casino is forbidden by law to alter the payout chip. These chips are sealed with a tamper-evident seal and can only be changed by the state Gaming Control Board. Other jurisdictions may have different rules.
Slot machines common in casinos at this time are more complicated. Most allow players to accept their winnings as credits which may be "spent" on additional spins.
Often machines are linked together in a way that allows a group of machines to offer a particularly large prize, or "jackpot". Each slot machine in the group contributes a small amount to this progressive jackpot, which is awarded to a player who gets (for example) a royal flush on a video poker machine, or a specific combination of symbols on a regular or 9 line slot machine. The amount paid for the progressive jackpot is usually far higher than any single slot machine could pay on its own.
In some cases multiple machines are linked across multiple casinos. In these cases, the machines may be owned by the machine maker who is responsible for paying the jackpot. The casinos lease the machines rather than owning them out right. Megabucks may be the best known example of this type of machine. Megabucks Nevada starts at $7,000,000 after a jackpot. The new penny Megabucks video game has a jackpot that starts at $10,000,000.
Slot machines that are not linked to a large regional jackpot such as Megabucks usually have better payout percentages, as linked machines have to take into consideration the large jackpot amount into their payout percentage calculations.