An online game is a game played over some forms of computer network. At the present, this almost always means the Internet or equivalent technology; but games have always used whatever technology was current: modems before the internet, and hard wired terminals before modems. The expansion of online gaming has reflected the overall expansion of computer networks from small local networks to the Internet and the growth of Internet access itself. Online games can range from simple text based games to games incorporating complex graphics and virtual worlds populated by many players simultaneously. Many online games have associated online communities, making online games a form of social activity beyond single player games.

The rising popularity of Flash and Java led to an Internet revolution where websites could utilize streaming video, audio, and a whole new set of user interactivity. When Microsoft began packaging Flash as a pre-installed component of IE, the Internet began to shift from a data/information spectrum to also offer on-demand entertainment. This revolution paved the way for sites to offer games to web surfers. Most online games like World of Warcraft, Final Fantasy XI and Lineage II charge a monthly fee to subscribe to their services, while games such as Guild Wars offer an alternative no monthly fee scheme. Many other sites relied on advertising revenues from on-site sponsors, while others, like RuneScape, the games made by Artix Entertainment and Mabinogi, let people play for free while leaving the players the option of paying, unlocking new content for the members.

After the dot-com bubble burst in 2001, many sites solely relying on advertising revenue dollars faced extreme adversity. Despite the decreasing profitability of online gaming websites, some sites have survived the fluctuating ad market by offsetting the advertising revenue loss by using the content as a cross-promotion tool for driving web visitors to other websites that the company owns.