The world of Final Fantasy X
The Final Fantasy game series is a wildly successful RPG franchise produced by Japanese company Square Enix. Each installation of the game is different from the next, with different characters, worlds, and stories. As should be obvious, FFX is the tenth game in the series and is played on Sony's Playstation 2. It is the first game in the series to feature voice acting - and pretty shoddy voice acting, to be frank!
Final Fantasy X takes place in a world called Spira, the "spiral of death." The inhabitants of this world are plagued by the presence of a destructive creature called Sin. The people live in constant fear of Sin, and their only reprieve from it is the short Calm that lasts after summoners defeat Sin. Summoners are people who embark on pilgrimages across Spira, visiting temples and obtaining the powers of aeons - powerful summons - in the hope that they can obtain the Final Aeon, thought to be the "only way" to destroy Sin. However, Sin is always reborn not long after one summoner defeats it (at the cost of his or her own life).
Spira is ruled by the the religion of Yevon, which expressly forbids the use of machina (machines). With the exception of the Al Bhed, the people of Spira unquestioningly accept the commandments of the Yevon clergy, and believe that Sin exists because of the war between Bevelle and Zanarkand 1000 years prior to the start of the game. Machina was used during the war, and with the complete and total destruction of Zanarkand during the war, the people were taught to believe that machina were bad.
In reality, it turns out that Sin is basically a giant aeon, and the reason it comes back is because Yu Yevon, a summoner who lived during the war, posesses the Final Aeon of the summoner who defeats Sin and uses the aeon to become the next Sin. The religion of Yevon is a hoax - machina are not actually bad and everyone in the clergy is undead and everyone in the upper ranks is corrupt. Sin is programmed to destroy areas with high population and more advanced technology in order to prevent another Machina War from breaking out. Encased in his armor of Sin, Yu Yevon uses the fayth - citizens of Zanarkand - to summon Dream Zanarkand, which is where our hero Tidus hails from. In FFX, we find out that Tidus comes to the real Spira thanks to the newest incarnation of Sin: Jecht, who is also from Dream Zanarkand. He is Tidus' father and was a guardian of High Summoner Braska. Once Tidus is dragged into the real world, he meets the aspiring summoner Yuna, who is Braska's daughter. Since he has nowhere to go, he goes along with Yuna and her guardians, and that is how the game begins. Over the course of the story, the party learns about the truth behind the religion of Yevon.
Final Fantasy X-2
FFX-2 is the sequel to FFX, and should not be confused with FFXII, which is a totally different game set in a totally different world. At the end of FFX, Yuna defeats Sin successfully without dying, but Tidus ends up disappearing. The fayth stop dreaming as a result of Sin's destruction, and thus Tidus, as a resident of Dream Zanarkand, fades away. FFX-2 is Yuna's story in a post-Sin Spira, which is struggling to adapt to a new way of life. Yuna has become a spherehunter, so she travels across Spira looking for video spheres indicative of Spira's history. She hopes to find out more about Tidus and whether he is still alive.
This game wasn't very popular. It's a fun game, but a lot of people feel that it isn't an adequate sequel to the first game. I can see why they think so - Yuna's transformed from a prim and proper summoner to a much wilder and less-clothed person. Also, she doesn't have her aeons anymore, but a bunch of "dresspheres" which the party uses to change job classes in-battle.