Gambling Knowledge
You put coins in the machine (in an online casino you play with virtual chips (v-chips)) - pull the lever or push a button - the reels are set in motion. 
There are symbols mounted on the reels. The object of the game is to have all the symbols the same on the payline.
The number of reels, and lines can vary from machine to machine. The Symbols can be anything, there is no law that says they have to be cherries or lemons or lucky 7. 
Depending on the machine, you can bet up to three or four coins on each spin.
The most important thing is to read the rules. Slot machines are not all created equal.

Types of Slot Machines

There are two basic types: 
1.) Straight Slot Machines: Present Payoffs never change. Only when more coins are inserted, the payoff increases at a fixed ratio.
2.) Progressive Slot Machines: Jackpot becomes progressively larger as more coins are dropped into the machine. The jackpot continues to increase at a fixed percentage rate, based on how many coins are played over a long period of time.

There are eight sub-types:
1.) 1-line, 3-reel Machines: You win by lining up the proper symbols on one pay line.

2.) 3-line, 3-reel Machines: You win by lining up the proper symbols on any of three pay lines. The payoffs increase proportionally. The same symbols that would win you 20 coins on a one-line, three wheel machine would only pay ten coins on the first line, 20 on the second and 40 on the third.

3.) 5-line, 3-reel Machines: Pays off on the three horizontal and on two diagonal lines as well. Thus, you have a five-line play. 

4.) Low-level Machines: Have a built-in chair, so you can sit down comfortably with the machine at elbow level. These machines come in all the same standard types: 1-line, 3-line, and 5-line.

5.) Stand-up Machines: You either sit on a stool or stand to play.

6.) Pay-for-Play Machines: You only qualify for the wins associated with the number of coins you insert. One coin allows you to win on one line. Two coins allow you to win on two lines or twice the payout. 3 coins allow you to win on all 3 lines or three times the payout (depending on the machine)

7.) High-tech Machines: Only differ in appearance (Elvis Presley or Star Wars design or whatever). Payouts are the same.

8.) Progressive Jackpot Machines: Slots tied to banks, or carousels, of machines so that coins inserted will increase the jackpot of all the machines that are tied together. In Nevada there are machines that are tied together statewide. The Megabucks machine in Nevada starts the jackpot at $5 million.  

Payouts

25c machines pay out more than the 5c ones. Dollar slots payout more than the 25c machines, and $5 machines payout more than the $1 machines. 
Minimum payout in Nevada by law is 75%. Minimum payout in Atlantic City by law is 83%. Actual payouts are much higher. In Atlantic city they are 91% and in Nevada about 95%.

Pay attention to what machine you play. In this example you start off with $100 at each machine and you can see how fast you loose the money after 15 pulls.

Where to play

The best paying slots can be found in the most busy gambling areas, like the big casinos in Nevada, Mississippi, Colorado etc. and not in isolated areas or cruise ships. There are newsletters and monthly magazines that list slot machine payout percentages by state, area and individual casinos. 

Generally, the best paying slots are placed near the change boths, so people change more money when hearing lots of people winning, and near the coffee shops, so people eat faster to get back to the machines. Players are more likely to play when they see winners, so probably the loose slots are placed in areas that are highly visible. 

Tight machines are normally placed on either side of a loose one. When you find one that is paying off, don't play the ones on either side of it.
Further, tight slots are placed near the craps and blackjack tables, roulette wheels and sports book, because winning players make a lot of noise and would distract the other players. 

Random Number Generator (RNG)
A Random Number Generator is a computer program built into a slot machine, or that is part of the software that powers an online slot, that selects the outcome of a given pull of the arm. It does exactly what it’s name claims: it generates random numbers. These numbers are constantly being generated, even when the machine is not being played.

These programs are set according to the percentage of payout that the Casino has decided on. Although they can set any maximum they want, there is a minimum that is established by the Gambling Commission whose jurisdiction they are under.

Play the highest denomination machines that fall into your budget. The higher the denomination, the higher the payout.

Play slots at best locations, those that advertise high returns, and those that are listed in various publications as being the best. 

Always read the machine's payout table prior to inserting any money and always insert the maximum amount of coins. 

Play the coins in your hand / rack, and not the ones on the credits. This way you can track the game and leave the machine when you're winning. 

Divide your bankroll by days of stay and that by sessions per day to know how much you can spend per hour. Example: $1000 / 4 days / 3 sessions = You can lose $65 per session. 

Play 3-reel machines only. Chances to hit the jackpot are 8,000 to 1 for 3-reels, 160,000 to 1 for 4-reels, and 3,000,000 to 1 for 5-reels.

Join all the slots clubs offered by the casino. Joining is for free, and you will get back some of your investment in the form of comps. 

Play in slots tournaments. Entry is free and the wins are high. Ask for them wherever you go. They are fun and inexpensive.