Final Fantasy XIII
In the main FFXIII game, the world is split into two parts: Cocoon and Pulse. Cocoon is a space colony that houses a few million people. Pulse is everything outside of it.
For the people of Cocoon, Pulse is a frightening place. Those who come into contact with it are believed to be elements of a plot to invade Cocoon and are banished from the protections of the Cocoon realm by the “holy” government and its army. The original E3 trailer depicted a group of people who seemed to be confined on a train. These people had come into contact with Pulse and were being quarantined.
Cocoon is protected by crystals. This may not necessarily be the case for the rest of the games in the Fabula Nova Crystalis series, but the crystals in FFXIII are more than just shiny stones with magic abilities. They have the ability to create real life objects. They make life easy for people in Cocoon by creating machines and living creatures.
The relationship between Cocoon and Pulse can be thought of as this: first, there was Pulse, a dangerous world where people reside under the fear of monster attack. Cocoon was made when the crystals considered what would be an appropriate living space for people forced to reside in the world of Pulse.
FFXIII’s main character is known as Lightning, although that’s not her real name. A battle expert with mastery of gun, sword, hand-to-hand, and magic based combat, Lightning knows little about herself. In fact, she’s not even sure of her real name. Lighting was once a high ranking official in an organization of some form. Now, according to the prologue that kicks off the trailer, she’s been selected as an enemy to all of man kind and will bring destruction to the world.
Final Fantasy versus XIII
In FF versus XIII crystals have a different role in the Versus world, director Tetsuya Nomura explained to Famitsu. In Versus, crystals exist only in the main character’s country. A long cold war revolving around these crystals, has, as suggested by the news broadcast, at last begun to clear.
The various weapons in the game will have different methods of attack, and some will even be usable as shields. Weapons will have some sort of customization element. Of course, the main character won’t have his full arsenal available to him at the outset of the game.
Final Fantasy agito XIII
The game is set in a magic school named Akademia, and will gradually expand to the larger world outside of the school. It is described as an online RPG, but not "massively multiplayer".