Any game that allows more than one player to play is considered a multiplayer game. A multiplayer game is most often played over the Internet, however, could be also a game played over a network (LAN) or dial-up connection. Split screen multiplayer games are also popular with consoles for allowing multiple people in the same room play the same game (usually only between 2 to 4 players).
Most online multiplayer games connect players together by joining a multiplayer server, which lists all available online games and allow players to connect to a game or create a new game for others to join. Although a main game server is controlled by the developer of the game, many games also allow anyone to create and control their own game server.
Each game and game map is different so how many people can play at the same time varies, however, most games support between 5 and 50 players playing at the same time. Mass Multiplayer Online (MMO) games can support several thousand people playing at once.
In the below picture, is a great example of a multiplayer experience called a LAN party. Here, hundreds of gamers have brought their computers to a central location and connected them together over a network, allowing them to compete with each other.