Game Play - Final Fantasy XIII 

It's Final Fantasy, a Little Different

Knowledge about the gameplay of Final Fantasy XIII is basically limited to the battle system at the moment.
Final Fantasy XIII Battle Screen
Final Fantasy XIII's battle system is shaping up to be radically different from those of Final Fantasy's past. The main points of difference are: the battle system is designed to be as flashy, smooth, animated and fast passed as Advent Children's battle scenes, there is no MP in battles but an ATB Cost Gauge, and cost figures beside each command, magic and non magic alike, there are no random battles, but battles aren't as seamless as Final Fantasy XII's system, weapons can transform during battle if the player choses, different attacks can be sequenced together in a large variety of combinations, players have to be mindful of their position on the battle field, including vertically, and the final not worth addition is the inclusion of an OverClock mode.
Final Fantasy XIII OverClocked Guage
OverClock mode is reminiscent of the drive gauge from Kingdom Hearts or the Limit Bar from Final Fantasy VII. Once the gauge is filled (it is unclear what drives it to fill), players can enable the OverClock mode, which slows down all of the surrounding enemies, as well as unlocking special character specific abilities and increased attack and magic power.

 

Lightning, specifically, has small metal plates on her finger tips which allow her to manipulate gravity. Doing so enables her to preform some extraordinary acrobatics while enveloping her body with a blue-ish aura.
Final Fantasy XIII Lightning Aura
You will be able to fight with a party in Final Fantasy XIII, and control the other characters, however many are allowed in a party is currently unclear. More control is said to be had in the new battle system despite it's increased speed. Also the system is still time based, time still flows as you input commands, and you will have to wait for your turn during battle based on the ATB (Active Time Battle) Gauge.

The vision behind then new battle system is quite broad and difficult to follow as most of what has been learnt has left a very large amount of questions unanswered. The conceptualization of the system, which has been notes a bit publicly so far is vague and full of holes. Don't expect to fully understand the battle system until Square releases game-play videos of it in action.