Online casinos provide a website for players to go and place bets on casino games like Slot machine games or card games. It is a very popular type of online gambling and provides an outlet for players who wouldn't be able to visit land based casinos, of which there are far fewer. The online casinos offer percentage paybacks on the games that would be slightly higher in odds compared to the land based casinos. Each casino games has it's own payout percentage and you can find this information on the casino websites in their FAQ, Help sections.