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.

Online casino types

Online casinos can be divided into three groups based on their interface: web-based casinos, download-based casinos, and more recently live casinos. Some casinos offer multiple interfaces.

Web-based online casinos

Web-based online casinos are websites where users may play casino games without downloading software to the local computer. Games are mainly represented in the browser plugins Macromedia Flash, Macromedia Shockwave, or Java and require browser support for these plugins. Also, bandwidth is needed since all graphics, sounds and animations are loaded through the web via the plugin. Some online casinos also allow gameplay through a plain HTML interface.

Download-based online casinos

Download-based online casinos require the download of the software client in order to play and wager on the casino games offered. The online casino software connects to the casino service provider and handles contact without browser support. Download-based online casinos generally run faster than web-based online casinos since the graphics and sound programs are located within the software client, rather than having to be loaded from the Internet. On the other hand, the initial download and installation of a download-based online casino client does take time. As with any download from the Internet, the risk of the program containing malware does exist.

Live-based casinos

Live-based casino gaming allows web players to interact with games played in a real world casino environment. Online players can see, hear, and interact with live dealers at tables in casino studios worldwide.