A Guide To Effort Values (EVs) - By Bass

EVs, introduced in Generation 3, are values that make Pokemon stronger in its stats the more it trains. For a Pokemon to gain EVs, it battles against certain other Pokemon. It doesn't matter what the level is - what matters is the species. For example, defeating a Gyarados grants 2 Atk EVs. EVs are also not split; any Pokemon that got Exp. Points in that battle (this includes Exp. Share) all get two EVs. (Psypoke's Psydex has EV readings for all Pokemon.)
The maximum amount of EVs a Pokemon can have is 510, and a max of 255 per stat. EVs affect stats in the following way: You divide the total increase by 4, and ignore the remainder. This means a Pokemon with 100 DVs in Atk will have an Atk stat of 25 more at lvl 100 than the same Pokemon (with the same DVs) if the second one had 0 Atk EVs at lvl 100.

Any EV gain that goes over the 255 limit is ignored. Because of this, it's not necessary to train more than 252 EVs on a stat, since you'll always end up with +63. The EVs gained by the Pokemon have the stat growth distributed over level ups. It doesn't happen all at once.

(C) Bass - 2010