There are essentially two kinds of slot machines: flat tops or straight slots, and progressive slots. These slots are the same, except for one feature: flat tops will pay the winner a predetermined amount, whereas progressive slots have a jackpot that grows by a percentage of each coin played. In most cases now, a number of progressive machines feed into one jackpot where any machine in the group can win. The causes the jackpot to grow at an astronomical rate. These groupings can be a number of machines in one casino, or can be networked to all of the same machines in a whole state. Progressive machines can be identified by flashing electronic payoff signs displayed either on top of the machine or above a grouping. Today, slot machine networks span across the country and online slot progressives will span across the globe.

Apart from the 2 basic types, there are many variations and themes of slots, and whether playing a progressive or a straight slot, players will be faced with a number of choices. Machines vary on denomination of coins, the number of reels, how many coins to play, and single or multiple paylines, not to mention other special features.

Since the quarter and dollar machines are most popular, they can be found all across the casino floor. Nickel machines can still be found in small areas, depending on the casino. For the high-rolling player, casinos have instituted $5, $25, and $100 machines normally found in special VIP areas. These machines require the use of custom coin tokens or cash bills.

The most common machines have three reels, but casinos do offer machines with four or five. When it comes to the odds of winning what is most important are the number of reels a machine has, and how many symbols are displayed on each reel. Mathematically, the larger the number of reels and symbols, the harder it is to hit the jackpot or any other winning combination. The number of symbols on each reel is programmed into the computer that operates the machine, and the actual number of symbols depends on the size of the jackpot that the machine offers. The reason for the programmed symbols is to make the jackpot rare enough that the machine will generate enough money to pay for the jackpot and yield a profit for the casino. Most machines are set to pay out anywhere from 83 to 98 percent of the coins that are played in the course of a pay cycle.

On average, machines operate on a two or three coin maximum. When it comes to multiple coin playing machines, there are subtle variations. With some machines, the number of coins played is proportional to the payoff, and with others, the payoff is more than proportional. Some machines don't even pay a jackpot on one coin, and the three-coin jackpot often pays 150% of a two-coin win. In this case it is best to play the maximum number of coins.

The standard slot machine has one payline, often called the classic slot, but three paylines are not that uncommon. For multi-lined paylines, a coin has to be played for each payline in order to get paid for a win.

When players hit one of the smaller winning combinations they will be paid automatically by the machine. The larger jackpots will be paid out directly by a casino employee who is notified by a light on the top of the machine. Each casino's slot payoffs will vary.