Online casinos, also known as virtual casinos or internet casinos, are online versions of traditional ("brick and mortar") casinos. Online casinos enable gamblers to play and wager on casino games through the Internet.

Online casinos generally offer odds and payback percentages that are comparable to land-based casinos. Some online casinos claim higher payback percentages for slot machine games, and some publish payout percentage audits on their websites.

Assuming that the online casino is using an appropriately programmed random number generator, table games like blackjack have an established house edge. The payout percentage for these games are established by the rules of the game. Reliability and trust issues are commonplace and often questioned.

Many online casinos lease or purchase their software from well-known companies like Microgaming, Realtime Gaming, Playtech, and CryptoLogic Inc in an attempt to "piggyback" their reputation on the software manufacturer's credibility. These software companies either use or claim to use random number generators to ensure that the numbers, cards or dice appear randomly.