MMO
Games where thousands of players share a world that goes on existing after you log out.
The form descends from text-based MUDs on 1970s and 80s university networks and gained graphics with Meridian 59, Ultima Online in 1997 and EverQuest in 1999. The hard part technically is not the graphics but the persistence: a world changed by players must be stored, split across servers and protected against one player's actions ruining it for everyone else.
The social side became the genre's real content. Guilds, economies with their own inflation and outright power struggles have been studied by economists and sociologists alike, and virtual items have been traded for real money since the early 2000s. The subscription model has largely given way to free-to-play with in-game purchases.