To play Slots. Online or offline money or a credit card is deposited in the machine (your wager).
Your deposit is shown as playing credits. The reels are spun by clicking on the spin button,
when the reels come to rest you win or loose  in accordance with the pay out table and the casino rules.

Winnings can be either put towards your playing credit or cashed out.  The holding of symbols to the next spin is allowed when indicated  as is the placing of side bets (opportunities to increase your stake and receive higher winnings is the required combination of symbols come to rest), nudging (the opportunity to lower or raise symbols in order to achieve a winning combination on the reels - the number and direction of  nudges allowed will be indicated by the machine) and cashing out.

The pay out table is based upon combinations of various symbols.  All possible resting places (the stops) on the reels have been assigned  numbers,  when the spin is activated a micro chip selects a random number for each reel, the random number is divided by a number determined by the casino and thus becomes the predetermined resting place (stop) for that reel.  Traditionally the first reel is the most likely to show winning symbols and is deemed loose and the last reel shows the least winning symbols and is deemed tight.  To hit the jackpot is have the reels rest on the maximum pay out, as the selection of stops is random it is possible to hit another jackpot immediately after hitting a jackpot.  Better odds of winning occur when the maximum number of credits per play is used.

Progressive Jackpots are a new development.   Basically you are playing on machines that are linked together.   Because they are linked the jackpot grows faster and the pay outs are huge.   Progressive slots cater to all budgets and are great for getting a big return from a small investment.