Effort values (abbreviated EVs and previously called Stat Exp) are
attributes which give bonuses to a Pokémon's stats and improve
differently depending which Pokémon they defeat. These bonuses, in the
form of effort points, are gained in addition to bonuses gained by
increasing level- http://bulbapedia.bulbagarden.net/wiki/Effort_values
EV Wing items-
Health Wing (HP)
Muscle Wing (Attack)
Resist Wing (Defense)
Genius Wing (Sp. Attack)
Clever Wing (Sp. Defense)
Swift Wing (Speed)
Power Items-
Power Weight (HP)
Power Bracer (Attack)
Power Belt (Defense)
Power Lens (Sp. Attack)
Power Band (Sp. Defense)
Power Anklet (Speed)
Other- Macho Brace, Vitamins, Exp. Share, Pokerus
Check out our EV site(this helped me a lot and that's the reason i had became stronger at battling)-- http://pokemondb.net/ev
OR check this video(this helped me out too)-- http://www.youtube.com/watch?v=Jch784-3QUQ