Battle System
Here we have a complete analysis of the
Final Fantasy XIII Battle System, we can say it is very similar to that of Final Fantasy XI and
Final Fantasy XII except for it won't include the attack lines which FFXII uses.

The basic idea is you pick an attack from the "Attack Menu" and for each attack you fill up a slot, in the "Attack Slots", I presume that filling consecutive slots can lead to a combination of attacks but that hasn't been confirmed. Once your ATB guage has filled up you can execute the attacks in the "Attack Slots". Also when someone is attack it shows the effect on them (Attack Element) so if you used a move such as Fire on a character with Blizzard traits in theory it should come up something along the lines of "Super Effective".
It won't be a fixed battle system, so you'll be running around and attacking enemies, similar to how you do in
Phantasy Star Online. Also an interesting feature is the limit meter in the corner of the screen, once it fills up you will be able to use your "Limit Break" like in
Final Fantasy VII. We know this is a meter because in the screenshot below it is full and flashing:

Due to poor quality of the screenshot we can not tell if the character is glowing or if it's just a lack of quality but we do know however that the attack is over 20% more powerful when the limit guage is full.
All of the above is speculation and we can't say it's correct for deffinate, but for any more updates on the FFXIII Battle System make sure to check back here!