Nintendo has revealed that Pokémon X and Y will feature a new type of evolution unlike any that has previously appeared in the series. This "Mega Evolution" allows a Pokémon to temporarily transform into a much more powerful state in the middle of a fight, but the Pokémon will return to its previous form at the end of the battle. This transformation requires a rare item called a Mega Stone, and it appears that there is a unique Mega Stone required for each Mega Evolution.
So far, Game Freak is focusing on three Pokémon who have been shown to have Mega forms: Mewtwo, Lucario, and Blaziken (Mawile, Absol, and Ampharos have been shown in Mega Form in Japan's CoroCoro magazine). Mega Mewtwo is the previously unexplained new form of Mewtwo that was shown some time ago, while the other two were revealed today. Mega Blaziken cannot be acquired through normal gameplay, but a Torchic holding the Blazekenite Mega Stone required for that form will be distributed via the Nintendo Network starting on October 12, the same day as the game's launch.
Nintendo also revealed today that Pokémon X and Y will contain a new system called "Super Training." This will let trainers strengthen their Pokémon outside of battle through various training regimens and mini-games. Through this method, a Pokémon's base stats can be directly improved, though only a limited number of times.