MMO is the general term given to a wide range of games and it includes a variety of different genres; a Massive Multiplayer Online game is any game that can support a large number of players in the same world.
These game are extremely varied but there are a few typical features that they all share:
Every MMO genre has these features on some level however, there are a wide variety of genres and finding features they all share can be a difficult thing. Even games in the same genre can vary quite a bit and not all MMO games are easy to label as a certain genre.
MMO games are some of the most popular games around, especially on PC and mobile devices; these games are extremely easy to access and many are free to play. Free-to-play MMO games have large communities with alliances, guilds or teams and players are encouraged to work together to complete goals; not only this, but they offer high replay value and plenty of content for very little money.
MMO games are a varied but well-loved group of games that are played by people all over the world; players join each other in a single world and each have similar goals as they fight to progress through the game. They offer a lot of content, high replay value and a strong social community that's filled with passionate players.