The Worlds
The world of Final Fantasy XIII is largely split into two areas: Pulse and Cocoon. You'll be traveling between these two areas, although just how you'll get from one to the other is something you'll have to find out by playing the game for yourself.
We've listed just a sampling of the areas that Square Enix has introduced.
Cocoon
A utopia floating in the sky. The Japanese kanji for Cocoon is literally "Utopia." Much of this part of the Final Fantasy XIII world consists of high-tech cities, but there are a few natural areas, including the forest environment that was depicted in early trailers.
Hanged Edge: This is the city setting that was depicted in the Advent Children demo and is the first area that you'll encounter in the game proper. It's been sealed off by the government -- being the closest part of Cocoon to Pulse, it has taken the most influence from the lower world.

The Sunleth Waterscape: A rare natural preserve in Cocoon. The weather of this area is controlled by a fal'Cie. What this means in terms of gameplay is that you can modify the weather by accessing special terminals. You'll encounter different monsters depending on the weather.

Palum Polum: A major city in Cocoon and one of the rare places where people actually shop for things (online shopping is big in Cocoon). The city is now packed with guards protecting from invading l'Cie -- that is to say, you! Below Palum Polum, you'll encounter a fal'Cie who supports the city's food supply and acts as an artificial sun.

Nautilus: An amusement area with a government-run theme park. Nautilus Park is the setting for the fireworks display that, in past trailers, has featured a romantic scene between Snow and Serah. In addition to engaging in RPG-like banter with visitors of this area, you'll find farms with Chocobos and other animals. Monitors lining the park play back videos showing a war from hundreds of years back between Pulse and Cocoon.

Pulse
The massive "lower world" (that's the literal translation for Pulse's kanji) which spreads out below Cocoon. While Cocoon is considered a paradise for people, the natural Pulse world could be thought of as a paradise for monsters, which is why the beasts of Pulse are so big. Pulse's proper name is actually "Gran Pulse."

Mount Yaschas: This mountainous area appears to house the ruins of an ancient Pulse civilization. Now all it houses are zombie-like Cie Corposes (the remnants of l'Cie who fail to fulfill their focus) and a variety of monsters. Mount Yaschas doesn't have a direct role in the game's story, but you'll find rare large beasts here along with exclusive missions.

Combat
Final Fantasy XIII uses an encounter-based battle system. You can see your enemies in advance and avoid them if you choose.
Get into combat and you'll find yourself in control of just one character in a party of three. The other characters are controlled by the AI.
As with past Final Fantasy games, XIII uses some variation of the "Active Time Battle" or "ATB" system. This basically means that the action doesn't while you're selecting a command to input (those who are stressed out by such things will be pleased to know that Final Fantasy XIII lets you select a "slow" speed from the options menu to calm things down).
Final Fantasy XIII's ATB system puts a large multi-unit gauge above your character's command menu. Moves that you select from the menu are placed on this gauge based off their cost. Weaker attacks take up just one unit on the gauge, while others take up multiple units or even the full gauge. You're free to execute moves individually, but let the gauge fill up and all your selected moves will execute in one go.
To the upper right of the screen, you'll see a gauge with some percentages. The percentages represent a "Chain Bonus." String together attacks and you'll build this figure up. Cross the "Break" amount listed next to it and you'll end up in a Break state. You'll be able to beat up on your foes with greater force until the gauge above the percentages depletes itself.

Roles
The moves that you have access to on your command menu depend on your character's Role. Consider these "Roles" to be the XIII equivalent of the "Jobs" in other Final Fantasy games. Each role gives your character unique abilities along with status boosters for such areas as attack and defensive power.
The game includes a total of six roles, although each character only starts with three and acquires access to the other three as the game progresses.
The six roles are as follows:
Attacker
An offensive role. In fact, it can only perform attacks, so you'll need to have a different role set up for healing purposes. Attackers have access to such abilities as "Area Blast," and they receive bonuses for physical and magic-based attacks. Square Enix suggests combining Attackers with a Blaster for damage-heavy chain attacks.
Defender
Characters assigned to this role get boosts in physical and magic defense. In addition to absorbing enemy attacks in order to protect allies, Defenders can perform counter attacks.
Blaster
Blasters have access to abilities such as Fire, Blizzard, and Thunder, and are good with elemental-based attacks. They can combine attacks using different elements to improve the power of the technique.
Healer
Healers have access to healing abilities like Cure and are given role bonuses such as increased effectiveness for recovery abilities and items. Healers can also heal allies who have died in battle. However, they are unable to actually attack enemies.
Enhancer
Enhancers have access to status-changing abilities such as "Haste." They can also be used to increase the strength, defense, and other abilities of allies and can also give elemental properties to their ally's abilities.
Jammer
This role can reduce enemy attack and defense strength and can also cast status-inflicting spells on its opponents.
If you're big on RPGs, you can probably see some similarities between these roles and classic Final Fantasy jobs like Warrior and Mage. So why did the development team refrain from using the name "Job" and go with unique names? Director Motomu Toriyama and producer Yoshinori Kitase explained in past interviews that the borders between Final Fantasy XIII's roles differ when compared to the jobs of past Final Fantasy games. An Attacker, for instance, can use both sword-based and magic-based attacks, making it different from what one might consider the obvious analogy to the Warrior job.

Paradigms / Optimas
You don't directly assign a role to a character. Instead, you set up Paradigms: groupings of roles to be applied to each character in your party. In Japan, Paradigms are known as "Optimas."
During battle, you can change the capabilities of your entire party by switching between Paradigms. Pressing L1 brings up a menu that lets you select from some Paradigms that you've set up in advance. You'll want to plan ahead to make sure you have a good set of Paradigms at your disposal, then switch between them depending on the current battle situation.
The various Paradigm combinations have proper names like Rush Assault, Assault & Cure, and Delta Attack (these can't be customized). Not all role groupings form a useful Paradigm, though. In an interview, Kitase explained that players wouldn't want to use a Paradigm consisting of three Attackers. You might, however, use one that consists of three Blasters.
The availability of just six roles may seem a bit low, but when considering all the Paradigm combinations, Toriyama has said that he believes Final Fantasy XIII's battle system will be more varied than past franchise games.
You'll find yourself gaining access to additional Paradigms as the game progresses. These are added because your characters gain access to new roles, leading to the availability of additional combinations for your entire party.
Eidolons and Gestalt Mode
Roles and Paradigms represent a marked difference from past Final Fantasy games. But the change is nothing compared to the game's handling of summons.
As detailed on the character page, each character has a personal Eidolon. You can call the Eidolon out into battle when you need some extra power. The other two members of your party disappear and the summon takes their place, fighting under the control of the CPU.

You can take direct control of the summon using Gestalt Mode. This is known as "Driving Mode" in the Japanese version of the game because the summons transform into vehicles and you ride them. Here, you have direct control over the attacks the summon makes, selecting from five different attacks. The attacks have different costs, some bringing an end to Gestalt Mode after just one use.
The game's Tokyo Game Show build allowed players to call summons and switch to Driving Mode without end. There are limitations in the final game. In an interview, Toriyama said that most players will probably be able to call a summon once every 10 battles, although this of course will differ depending on play style.
Keep reading to see how you'll customize your Final Fantasy XIII play experience, as well as learn some more gameplay systems that could suck up your time.
"I may not be a mathematician, but I can count to a million." - Shannon Sharpe