The world of Final Fantasy XI is known as Vana’diel. It consists of two main landmasses with two smaller islands flanking them, which in turn are surrounded by small islands. It features diverse climates, ranging from the northern glaciers to the southern deserts. The four main cities in Vana’diel are Bastok, San d’Oria, Windurst, and Jeuno. The expansion Treasures of Aht Urhgan added the large Aht Urhgan Whitegate/Al Zahbi city area. The rest of Vana’diel is made up of a number of outdoor, dungeon, and minor town areas split into various regions. While most areas are accessible by walking, various modes of transportation, ranging from the classic Final Fantasy Chocobo and airships to special spells, facilitate movement across the game world.
The story is set in the fantasy world of Vana’diel, where player-created avatars can both compete and cooperate in a variety of objectives to develop an assortment of jobs, skills, and earn in-game item rewards. Players can also undertake an array of quests and progress through the in-game hierarchy and thus through the major plot of the game.
The events of the game are set 20 years after the Crystal War, when the nations of San d’Oria, Bastok, and Windurst on the main continent of Vana’diel fought and defeated the Shadow Lord and his army of beastmen. A parallel world named Dynamis, in which the beastmen succeeded in conquering Vana’diel, can also be explored. It is described as a dream world created by the Avatar of Dreams, Diabolos.
Taken from Wikipedia.