An online casino is a website or app that offers casino games for real money. In regulated markets it needs a licence from the national regulator, which checks that games are fair, that player money is protected and that tools like deposit limits are available.
RNG and RTP
Slots and digital table games use a random number generator (RNG): every spin or deal is independent. The return to player (RTP) is the share of all stakes a game pays back over millions of rounds, for example 96%. Over a short session anything can happen; over time the house keeps the difference.
Live dealers
Live tables stream a real dealer from a studio. The rules are the same as in a casino; you place bets on screen.