Effort values, you can learn more at http://bulbapedia.bulbagarden.net/wiki/EV
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EVs stand for Effort Values. They are how much of a stat your Pokemon earns when it levels up. If you wish to work on a single stat, fight certain Pokemon. If you level up by fighting a lot of Starlys, you will get lots of the speed EVs, giving your Pokemon more of the speed stat. Those items like Proetin and Iron also give you EVs. If you EV train your Pokemon a lot they will become very powerful.
Metagross "Attack boosting nature 252 evs speed, 252 evs sp. defense and 6 evs attack with thunderpunch, meteor mash, zen headbutt and hammer arm with leftovers (Given to you) Garchomp "Attack boosting nature 252 evs speed, 252 evs attack, 6 evs sp def with dragon claw, earthquake, outrage and firefang. with Yache berry (You find while enroute) Tyranitar "Attack boosting nature 252 evs speed, 252 evs attack, 6 evs defense. thunderfang, crunch, stone edge and earthquake with wide lense (the Veilstone gamecorner)
Max out your Pokemon's EVs.
Yes, the Pokémon still gets everything the Pokémon that defeats the enemy Pokémon gets which includes Experience Points and EVs.
One to three without items and Pokerus, up to, I think, eight with Pokerus and the right items. Go to Bulbapedia if you want to know which Pokemon give you which EVs.