EV stands for Effort Values. EV's can make a pokemon stronger by boosting their stats (attack defense ect.) higher and faster. For example when you battle a zubat you get 1 EV point in speed. You need 4 EV points in one stat to get an extra plus one to that stat on your next level up. So if you battle 40 zubats, and leveled up, you would get an extra +10 you speed. (correct me if I'm wrong) and theirs other stuff like the limit of EV's you can get per stat or pokemon or something. I don't remember that stuff.
Hope this helps!:)