Verified answer

EV training is simple.

Each time you KO a Pokemon, you get EVs. You get EVs whenever you get experience. EVs are assigned to a particular stat, meaning that there's different EVs for each stat.

The species of Pokemon you KO determines the quantity of EVs (usually 1-3) you obtain and for which stat(s). The EV yield of each species of Pokemon is constant, meaning that KOing the same species of Pokemon over and over will always yield the same EVs for the same stat.

EV training is doing just that, hunting the same species of Pokemon over and over to farm EVs for a given stat.

Let's talk numbers: For every 4 EVs in a stat, the Pokemon will have 1 more point in that stat at level 100, the lower the level of the Pokemon, the more EVs are needed to impact the stats. A Pokemon can have a maximum of 255 EVs in one stat, and a total of 510 EVs across all stats, allowing for two stats to be maxed.

There are items and other such things that can work to promote EV gain. The Macho Brace doubles all EV gains for the holder, but the holder has its speed halved while it is holding the Macho Brace. The Power items give the holder +4 EVs to the stat it promotes, even if the target does not yield EVs for that stat. The Power items also halve the speed of the holder for as long as it holds it.

Now there's one last thing that helps EVs... Pokerus. Pokerus is a condition that is applied to a Pokemon when battling other wild Pokemon with it. It spawns on wild Pokemon at about 1/4th the chance of finding a shiny, so you can also just catch a Pokemon infected with it. There's no visual indicator of whether a Pokemon has Pokerus or not outside of looking at the summary page. The best thing about Pokerus is that it stacks with the EV growth items. Pokerus+a Power item is the best setup, netting you 12 EVs per KO if the Power item gives the same EVs as the Pokemon you KO.

Now, things you would want to EV train for...

You would benefit from EV training when doing Battle Frontier. If you are playing competitively with other players, you would also greatly benefit. EV training is pretty much required for that.

When defeated, every pokemon gives off a certain number of effort values in a certain stat. For example, say your geodude defeats a pidgey. When the pidgey is defeated, geodude receives one speed EV, because that is what pidgey gives. Once geodude reaches ten speed EV's, his speed stat will go up one point.

Added note: each pokemon can only receive 510 EV's in it's life, and it can only receive 255 EV's of a singe stat.

Seeing as I have a lot of trouble telling the difference between EV and IV (as my memory in that field is pretty bad), I'm providing a link to JWittz's EV and IV Explained video.

He explains it in a simple and engaging way.

http://www.youtube.com/watch?v=1u_37RK-mFA&list=PL...