This is the same as pulling the arm on a real slot machine. Where the wheels stop determines if you win and how much. Each game will have its own pay table. For example, three bars in a row may pay 20 to 1. Three cherries in a row may pay 3 to 1.
Some machines have three or five paylines, or pay on the diagonal.
Check the casino rules for details. The result of each spin of the wheels is determined by a computer program that generates random results. This is true for both online slots games and slot machines found in hotel casinos. The machines can easily be set by the casino to generate whatever payout they choose. That means for every 100 dollars bet, the machine will, on average, pay out 95 dollars.