Gardevoir's evolutionary chain is somewhat unique as it is only shared by a small number of Pokémon. That being this Pokémon starts out as a Ralts and will evolve into Kirlia upon reaching level 20. From here there are two choices: you can evolve Kirlia into a Gardevoir if leveled up to level 30 or you can evolve Kirlia into a Gallade, a Fighting/Psychic-type, if it is male and is exposed to a Dawn Stone. In Pokémon X/Y and Pokémon Omega Ruby/Alpha Sapphire Gardevoir can evolve once more by using Mega Evolution.
If you are unfamiliar with Mega Evolution it is something that was introduced in Pokémon X/Y that allows a Pokémon to evolve once more, giving Pokémon battles a whole new twist. If the trainer has a Mega Ring (XY) or a Mega Bracelet (ΩRαS), a high friendship with the Pokémon, and the appropriate Mega Stone as their held item, a Pokémon can use Mega Evolution. While Mega Evolved a Pokémon can gain in power, access to a new ability and sometimes even changes to their type, creating a whole new playing field. However, unlike normal evolution it is not permanent and the Pokémon will revert back to its normal form once the battle is over. Also, if a Pokémon has already used Mega Evolution in a battle another Pokémon who can Mega Evolve will not be able to and the icon will be grayed out.
Another thing too is Mega Evolution is limited to a small number of Pokémon. Meaning that not just any Pokémon can Mega Evolve. Good thing for us Gardevoir is one of those Pokémon, eh? With a Gardevoirite as its held item Gardevoir can Mega Evolve into a Mega Gardevoir during battle. In Pokémon X/Y you can get one from a Ralts you'll receive in a trade with Diantha at Café Soleil in Lumiose City (you will need to be the Champion and have upgraded your Mega Ring first, however) and in Pokémon Omega Ruby/Alpha Sapphire you can get one from Wanda in Verdanturf Town after having defeated/captured Groudon or Kyogre.
Besides looking absolutely gorgeous Gardevoir gets a boost to several of its stats, most notably its Special Attack, Special Defense, and Speed which all get huge boosts. Its Attack gets a boost too but I usually ignore that stat anyway. Mega Gardevoir also gains the Ability Pixelate which turns Normal-type attacks into Fairy-Type attacks, which will increase their power as it is a Fairy-type Pokémon itself. I hear Hyper Beam is a good move to teach Gardevoir because of this, but I've never actually tried it. Mostly just because I don't like teaching Hyper Beam to my Pokémon due to them needing a turn to recharge.