EVs in pokemon Black/white?

Pokemon start off with 0 EVs in all stats. They gain EVs through battling different kinds of Pokemon. Each individual stat cannot have more than 255 EVs. A single Pokemon can only have a total of 510 EVs. After that, any battle the Pokemon is in will not affect the Pokemon's EV total. So let's say your Oshawott has 255 EVs in Attack and 255 EVs in Speed. (255+255=510) If that Oshawott were to battle another Pokemon, it would not gain any EVs because it has reached the maximum total.