Final Fantasy XI, also known as Final Fantasy XI Online is the franchise's first MMORPG (Massively Multiplayer Online RPG). Final Fantasy XI was the world's first cross-platform MMORPG, currently available for Windows, PlayStation 2, and Xbox 360.
Due to the nature of online RPGs, Final Fantasy XI differs from previous installments of the series, in that you are not the only hero. Instead, each player creates a customizable character and adventures through the world of Vana'diel with other players; leveling up, exploring, completing missions, quests, and much more. Like most of the gameplay, levelling up requires a group of people working together in what is called a 'Party', each contributing their own skills to help take down the foe and gain experience points. Battles occur in real-time, taking the ATB system to a new level, which was emulated again to a certain extent in Final Fantasy XII.
Currently there are some five hundred thousand people playing FFXI, utilizing over two million characters. With constant updates and Expansions added to the game, Vana'diel is not a static world; instead, it changes and evolves with the passage of time.
- http://finalfantasy.wikia.com/wiki/Final_Fantasy_XI