Massively Multiplayer Online games (also known as MMOs) are played online with many people, sometimes with hundreds of players.  There are many genres of these games from Casino, Role-Playing (mmorpg), First Person Shooter (mmofps), to Real-time Strategy (mmorts) and others.  These games have a persistent world, meaning it is active all the time.  There are often in-game events that bring players together to earn achievements, special in-game items, prestige and honor.  Events vary greatly for each game so join the game, invite your family and friends then have a blast!