Pokemon X and Y will provide players with new ways to boost their Pokemon's power. A Mega Evolution is a new type of transformation that will greatly increase the power of certain Pokemon.
Mewtwo and Lucario are some of the Pokemon able to undergo a Mega Evolution. This transformation occurs during battle "under certain conditions" and must be spurred by items called Mega Stones. Each Pokemon capable of a Mega Evolution has their own specific Mega Stone that must be found in the Kalos region.
When a Pokemon undergoes a Mega Evolution, their looks will change and their stats will increase. You can see Metwo's transformation in this video. Unlike normal evolutions, Mega Evolutions aren't permanent. The Pokemon will change back to normal once the battle is over.
One of the Mega Stones can't be earned by playing X and Y. In order to get Blazikenite, you'll have to take part in a special Wi-Fi event over Nintendo Network. The Blazikenite Mega Stone will come with Torchic, a Fire-based Pokemon that was available as a starter in Ruby, Sapphire, and Emerald. This specific Torchic comes with a Speed Boost ability that increases his speed after every turn. Also, once you level this Torchic up to 36, it'll evolve into a Blaziken that you can then Mega Evolve with the Blazikenite. The Wi-Fi distribution event will begin on October 12th, the global release date for X and Y
If you're looking to beef up your Pokemon, you'll have two training methods to use outside of battle. In Super Training, Pokemon will face off against Balloon Bots in virtual reality. Players will control their Pokemon in these matches, using the circle pad and touchscreen to fire balls into the Bots' goal while also dodging enemy shots. This activity will boost the base stats of the Pokemon over time.
Completing Super Training regimens will unlock training bags for Core Training. Pokemon in Core Training will work out on their own and raise their stats over time. It seems like it'll be a very handy way to keep your entire lineup of Pokemon in fighting shape.
X and Y developer Game Freak provided a trailer for the Mega Pokemon, which you can check out above. They also included images of these Mega Evolutions and the new training programs.