MMORPG or MMO is short for massively multiplayer online role playing game. It is a video game where thousands of people play together in an online world. In the game, the player starts at a low level or rank and progress until he or she has achieved the maximum level or rank. During a players lifetime they gain new abilities, skills, better gear, and get access to advanced areas in the game that often require a party, group, or raid of people to complete.
MMO games are also considered a social MMO because of the ability to chat with each other through private chat and public chat that all players can see and participate in if they so desire. The social aspect allows players to talk to other players, who often don't know each other and allow for role playing or casual chat with little to no consequences for what is said. The open nature of public chat can lead to offensive comments being made or players making fun of other players. However, many of these games provide a way for players to hide or turn off the public chat channels if they choose not to see it or chat with other players.
Some great examples of MMO games include Anarchy Online, EverQuest, Guild Wars, Runescape, Ultima Online, and World of Warcraft.