If I need to explain to you what a slot machine is then you probably shouldn't be going anywhere near a casino, but just in case: a slot machine is a device where you put in money, pull a handle or push a button, and it spins reels (either mechanical or video) that have a variety of symbols on them. If the symbols match up in a pre-determined way then you win money. If they don't, you don't. It's that simple and much more complicated all at once.
Slot machines these days are nothing more than a computer chip, which decides through a random sequence whether you've won or lost as soon as you push the button. This computer chip is called a Random Number Generator (RNG) and it sits there constantly spitting out strings of digits that equate to a loss or a win on the slot machine. The RNG has the capability to generate thousands of numbers a second and each one means something different in terms of payout.
When you push the "Spin Reels" button (or "Bet" button before you pull the handle), the RNG stops on whatever number it had generated at that exact split second. That number is the determination as to whether you've lost or won and if it's the latter, how much. Everything else - the handle, the reels, the bright lights, the sound - is just for show.
That's the easy part. The complicated part is the variation on the theme. Slots accept everything from pennies to $100,000 tokens although the most common are quarter, dollar, nickel, and multi-line penny machines. All machines accept as few as one coin and may take hundreds at a time (in the case of modern penny slots) but a limit of two or three is common.
By the way, when I speak of types of coins, I'm speaking in terms of a monetary unit and not actual coins. Most modern slot machines do not accept or payout in real coins anymore. They have mostly been replaced with a system that accepts bills of various denominations and pays out with a paper ticket that can be put in another machine or redeemed at the cashier window or change booth. Casual gamblers miss the rain of coins into the metal bin but most people have embraced the new system as easier, cleaner, and more convenient.
There are two basic types of machines: flat-top and progressive. A flat-top machine has a fixed amount that you can win if you are lucky enough to get the right combination on the reels. Amounts vary, from 2,500 coins to 25,000 and beyond.
A progressive machine works sort of like your local lottery. As people pump more money into it, the top "jackpot" grows until somebody gets the correct combination. Most are linked to a group of machines in the casino or even around the city or state and the jackpots can be huge: the largest single jackpot ever won was over $39 million on the Megabucks slots.
Individual progressive machines, where each unit has its own progressive jackpot not linked to any other machine, are very common.
Beyond this, the variations are endless. As gaming has developed, so have the slot machines that offer a variety of side games or bonuses that try to lure gamblers. There are "Wheel of Fortune," "Jeopardy," "Family Feud," and "The Price is Right" games that have similar gimmicks to the popular TV game shows they are modeled after. There are "Monopoly" and "Yahtzee" games that allow you to play the famous board games while you gamble. Movie, TV, and music themes are everywhere from Elvis to Frank Sinatra. Some have pinball games attached while others offer a variety of arcade style video games as bonuses.
About those bonus spins, reels, or games... in most instances if you have to take some action to initiate the game, it is a totally new number on the RNG. If the bonus starts automatically without you having to do something then it is part of the original win factored by the RNG when you first spun the reels.
Machines these days can be multi-denomination (meaning you can change from quarters to dollars for instance) and multi-game, where you can switch the theme. The latter is part of a technology called server-based gaming. In effect the slot machine is like a blank slate and you can "download" a variety of games from the main servers.
Before you sit down at any slot machine be sure to read the front of it carefully. It will explain everything you need to know about how to play it.
Don't let anybody try to tell you that there is some technique to winning at the slots. It's all random luck. But here are a few "rules" I try to live by when playing that don't always work but do often enough that make me remember them.
First, try to find a group of machines where lots of people are playing and winning. The casinos can increase the payouts on certain groups of machines at certain times so generally speaking, if you see an area of the casino where no one is gambling or winning, there's probably a reason. Find an empty machine next to someone with a lot of credits on their meter and sit down - you may be just as lucky.
It is an old wives' tale that machines on aisles or near the doors pay better than those buried in the casino. And yet, I've won more often on machines in those locations than I have elsewhere (and trust me, I pay attention to stuff like this). The theory behind it is just basic PR as the casinos try to get the people walking by to see people winning in the hopes it will lure them to play as well. Whether or not you believe its true will most likely depend on your future experience.
You can also try visiting the slots near a showroom just after a show or near the restaurants or buffets near mealtimes. This is the same concept as above where the casinos hope to lure people coming out of the shows or restaurants into the casino.
Finally, and most importantly, don't keep dumping your money into a machine that isn't paying you back something. Despite that sure-feeling you've got that it's about to hit big, it probably won't.