A massively multiplayer online game (also called MMOG or simply MMO) is a video game which is capable of supporting hundreds or thousands of players simultaneously. By necessity, they are played on the Internet, and feature at least one persistent world. They are, however, not necessarily games played on personal computers. Most of the newer game consoles, including the PlayStation Portable, PlayStation 3, Xbox 360, Nintendo DS and Wii can access the Internet and may therefore run MMO games.
MMOGs can enable players to cooperate and compete with each other on a large scale, and sometimes to interact meaningfully with people around the world. They include a variety of gameplay types, representing many video game genres.
source: wikipedia.com
MMO games, as the name implies, are multiplayer games. Which means you get to play with other players on the net and interact with them. There are a lot of MMO games on the net that you can play for free.
