EV's are a shortened version for Effort Values. Every time you feed your pokemon vitamins or level them up they earn EVs. Every pokemon gives different types of EVs. Ex: Gastly gives 1 Ev for special attack and cloyster gives 2 defense EV's. The more EVs a pokemon has in a single stat the stonger the pokemon is. You can learn more by asking other members or searching on Bulbapedia.