Effort Values (EVs for short) are what help determine what stats your Pokemon gain when they level up. EVs are similar to EXP in that your Pokemon will gain them when the battle finishes, but there are three key differences.
•Wild Pokemon give off a specific number of EVs.
•EVs will boost stats, as opposed to EXP, which only grows your Pokemon to its next level.
•There is a limit to the amount of EVs your Pokemon is capable of gaining, which has very little to do with the level it's at.
As the first bullet point says, wild Pokemon give off a specific number of EVs. Let's start with an example: you've started the game with a Snivy and just recently fought a Lillipup you found in the grass. Along with the 20 or so EXP points your Snivy gained, it also gained a EV point in Attack. Why? Because Lillipups give off 1 EV point in Attack when you defeat them. Each Pokemon gives off a certain number of EVs in a certain stat.
Pokemon will have higher stats if they have more EVs in the respective stat. At level 100, every 4 EVs that a Pokemon has gained in a stat will raise that stat by 1 point.
You don't have to wait until level 100 before you see the effects of EV training — you'll receive a proportionate bonus based on your level, and it is usually gained gradually, not all at once. In other words, you might not see every level up giving you +5's and +6's for your stats, but over time, your Pokemon will be stronger than it would be if you weren't EV training it.
There is a limit to the amount on EVs you can gain, which brings me to the third bullet point: You can only have a maximum of 510 EVs on one Pokemon, and no more than 255 on any stat. And since 4 EVs are worth 1 stat point at level 100, and 4 does not go equally into 510 or 255, then you don't have to completely fill up those numbers.
I just copied and pasted from Marriland.
For more go to:
http://pokemon.marriland.com/black_white/how_to_ev_train
