Slot machines, not specified
Slot Machine (also called Fruit machine) is a gambling game which consists of a few spinning reels that display various symbols. The player of the slot game needs to place bet on pay-lines and spin. If the combination of symbols on the reels match to the winning rules then the slot pays out according to the pay table.
Most games have a variety of winning combination of symbols, often posted on the face of the machine (or available on a different screen, accessible by touching a button on the main touchscreen, on video slot machines). If a player matches a combination according to the rules of the game, the slot machine pays the player cash or some other sort of value, such as extra games.
Reels
Until recently all slot machines used revolving mechanical reels to display and determine results. Although the original slot machine used five reels, simpler, and therefore more reliable, three reel machines quickly became the standard. The problem with three reel machines is that the number of combinations is only cubic - the original slot machine with three physical reels and 10 symbols on each reel had only 103 = 1,000 possible combinations. This limited the manufacturer's ability to offer large jackpots, since even the rarest event could happen every 1,000 plays. The maximum theoretical payout, assuming 100% return to player would be 1000 times the bet, but that would leave no room for other pays, making the machine very high risk, and also quite boring.
Video slots
Video slots are a modern innovation, with no moving parts at all - instead a graphical representation of one appears on screen. Since the player is essentially playing a computer game, the manufacturers are able to offer more interactive elements, such as advanced bonus games and advanced video graphics.
In addition, because there are no mechanical constraints on the design of video slots, most machines display five reels rather than three. This greatly expands the number of possibilities - a machine can have 50 or more symbols on a reel, giving winning probabilities as high as 300 million to 1 against - enough for even the largest jackpot. As there are so many combinations given by five reels, the manufacturers do not need to weight the payout symbols (although some may still do so) - instead, higher paying symbols will typically appear only once or twice on each reel, while more common symbols, earning a more frequent payout, will appear many times.
Video slots typically encourage the player to play multiple 'lines', so rather than simply taking the middle of the three symbols displayed on each reel, a line could go from top left to bottom right, or any of the other patterns specified by the manufacturer. As each symbol is equally likely, there is no difficulty for the manufacturer in allowing the player to take any or all of the possible lines on offer - the long-term return to player will be the same. The difference for the player is that the more lines he plays the more likely he is to get paid on a given spin - though of course he is betting more in the first place.