A recent developer blog post by Square-Enix's Tetsuya Nomura revealed a great deal of new information regarding the Fabula Nova Crystalis project, which comprises the PS3 games Final Fantasy XIII and Final Fantasy Versus XIII, as well as the cell phone game Final Fantasy Agito XIII.
The world of Final Fantasy XIII is called "Cocoon," and it uses crystal technology to create a highly advanced society. It is also completely surrounded by a large shell, which protects Cocoon and its inhabitants from all manner of evil residing outside of the shell, whose space is collectively called "Pulse." Cocoon's government reacts to the threat of the Pulse lurking monsters with "drastic measures," and uses the threat of exiling Cocoon citizens to Pulse as a means of controlling the people. Lightning, FF XIII's protagonist, is "chosen by the Crystal" to stop "those who would destroy the world," but Nomura (who is working on FF XIII as a character designer) was vague on any details otherwise.
Summons are going to be handled very differently in Final Fantasy XIII than in past FF games. They still serve as allies of our protagonist, but have some very unusual abilities; for example, the summon Shiva can now be transformed into a motorcycle, which Lightning can ride during battle. Somehow, this action-oriented feature is going to be implemented in a classic ATB system, but Nomura also stated that the game's developers are having "difficulties" implementing these new features.
Final Fantasy Versus XIII, an entirely separate game on its own, is supposedly set in a single kingdom within the same universe as Final Fantasy XIII and features a conflict centered over the last remaining crystal of said kingdom. The still-unnamed main character is a silver-haired swordsman who can teleport to any location where his sword is thrown. FF Versus XIII is being directed by Nomura himself and will use an action-oriented game engine based on the Kingdom Hearts series. Neither FF XIII nor FF Versus XIII has received a release date yet.
Final Fantasy Agito XIII is going to be a fully-3D cell phone game that centers on protecting another one of these crystals. That's essentially all that is known about Agito thus far, that and the fact that it doesn't have a release date either.