The rules for slot machines are quite simple. Each slot machine will have a line or set of lines across the window. If a certain combination of symbols falls on one or more of the lines when the wheel stops the player wins.
Slots generally fall in to one of two categories: straight slots and progressive slots. A straight slot machine pays out winning based on a structured payment agenda. Progressive slots however are totally random and follow no structured payout agenda, meaning that the jackpot increases as more and more people loose. If a jackpot is not hit in a reasonable period of time it should be very large in comparison with any straight slot jackpot. The lure of instantly winning a huge jackpot is why many gamers prefer progressive slots.