Warning: Spoilers follow for both Final Fantasy X-2 and Final Fantasy X
The game is set 2 years after the defeat of Sin and the ending of "Final Fantasy X", the game's obvious predecessor. The story revolves around three girls, the returning main characters Yuna and Rikku, and a new girl, named Paine.
Shortly before the beginning of this game, a Sphere was brought to Yuna's attention, which contained what appeared to be an image of her lost love, Tidus. Once again, his name is never said in this game, but the word used often by Yuna, "Kimi" has a special significance in Japanese. Apparently Kimahri found this Sphere on Mt. Gagazet, and gave it to Rikku. Once viewing it, Rikku rushed to Besaid to show Yuna. These events can be viewed on a special prologue named "Eternal Calm, Final Fantasy X-2: Prologue".
The world of Spira has basically been split into 3 groups: The Youth League, New Yevon and the Machine Faction. The Youth League is headed by Mevyn Nooj, New Yevon by Baralai and the Machine Faction by Gippal. Early in the game you are given the option to "side" with either the Youth League or New Yevon, which can slightly change some parts of the game.
The battle system from the previous game (CTB - Conditional Turn-Based) has been dropped in favour of a fast-paced variation on the traditional Active Turn Based system. Instead of waiting for enemies to attack, it is now possible to interupt an enemy whilst they are charging their attack. This also comes with it's disadvantages, because the same can be done to members of your party.
The field system has been upgraded, and now Yuna has the ability to jump and climb, and, in the immortal words of Ken Berry (SE Product Supervisor), Yuna can also "do heaps of other cool stuff". The field is still totally 3D, and many locations have returned from the previous game, with a number of new locations. Over the 2 years, Kilika has been rebuilt and looks simply amazing.
All of the music has changed, as Nobuo Uematsu has been temporarily replaced by Noriko Matsueda and Takahito Eguchi (composers for The Bouncer). There are many spectacular pieces in the game, and quite a few spectacularly tacky pieces.
The game also includes an amazing amount of side-quests and minigames, including Gunner's Gauntlet, the Den of Woe and the Via Infinito, amongst a lot of optional bosses who provide much of a challenge.
The game was released in Japan on March 13th, 2003, NA on November 18th, 2003 and is currently awaiting a PAL release.
An extended version of the game, entitled "Final Fantasy X-2 International + Last Mission" is to be released on February 19th, 2004.