Using the Sphere Grid
Unlike previous "Final Fantasy" games, there are no experience points or character levels in "Final Fantasy X." Instead, the game uses a feature called the Sphere Grid for advancement. Characters gain new abilities and attributes by earning Sphere Levels and moving around on the Sphere Grid. Check your characters' Sphere Levels often, and plot their Sphere Grid paths in advance. By planning ahead, you won't waste any Sphere Levels by taking a wrong turn on the grid.
Overdrives
Each character in "Final Fantasy X" has an Overdrive gauge next to his name. This gauge fills during battle, enabling the character to perform a special attack. As you collect new Overdrive attacks during the game, use them during battles to get a feel for how they work. If you have an especially difficult boss fight coming up, fill your characters' Overdrives before the battle to increase your offensive capability.
Aeons
Aeons are powerful allies that Yuna can summon during battle. Each of these Aeons has special abilities, and can gain attributes just like the rest of your party. You will collect new Aeons during the course of the game by completing the Cloister of Trials in each village. Use your Aeons often, and familiarize yourself with their abilities. They can be a powerful offensive tool in your arsenal, especially at higher levels.
Battles
In "Final Fantasy X," you'll encounter random battles and boss fights as you progress through the game. To be successful at these battles, play to your characters' strengths. Wakka, Auron and Kimahri are physical fighters; Rikku and Tidus are quick and helpful for support magic; Lulu specializes in black magic and Yuna's strengths are white magic and summons. Swap your characters in and out during battle to get offensive contributions from everyone.
Stealing
Characters in "Final Fantasy X" can use the Steal ability to obtain items from enemies during battle. Initially, only Rikku possesses this ability, but other characters can learn it from the Sphere Grid. Many enemies carry rare items that you can steal. Increase your chances of success by obtaining the Pickpocket ability or the Master Thief armor later in the game.