Okay. every pokemon can have up to 510 EVs. but 255 in each stat (hp, attack, special attack, defense, special defense, and speed) for every 4 effort values a pokemon has, it will get one point in that stat. sothe highest it can go is 252, making 63 points for your pokemon (252 divided by 4=63) Every pokemon has a certain effort value that it gives you. for instace typhlosion gives you 3 special attack Evs for each one you efeat. so if I beat 4 typhlosions, I get 12 speical attack Evs,which mean 3 extra points in special attack. This 63 makes a huge difference in how powerful your pokemon is.
pokemon Ev list.- this tells what pokemon give what EVs http://pokemondb.net/ev/all
http://pokemondb.net/ev general info.