Pokémon/Competitive battling/Effort Values (EVs)

First, please note that EVs only appear in the Third Generation. EVs are hidden values that can increase your Pokémon's stats. You can obtain EVs for every stat (HP, Attack, Defense, Special Attack, Special Defense, and Speed), with a maximum of 255 EVs for one single stat and 510 EVs total for all of the stats combined. For every 4 EVs of one stat, your Pokémon will gain 1 point to that stat. For example, if you gained 4 Attack EVs, then your Pokémon will gain 1 point in its Attack stat. It sounds great, right? How, then, do you obtain EVs to raise your Pokémon's stats?

Vitamins
Vitamins are HP Up, Protein, Iron, Calcium, Zinc, and Carbos. Each vitamin will give you 10 EVs of HP, Attack, Defense, Special Attack, Special Defense, and Speed, respectively. However, you can only use 10 Vitamins of each kind, with a total of 51 Vitamins can be used.

KO'ing a Pokémon
Believe it or not, fainting a Pokémon will gives you from 1 to 3 EVs of one stat, or the EVs could be split into 2 stats but the maximum EVs you can get total from one Pokémon is 3. For example, Aron gives +1 Defense EVs. If you fight a lot of Arons, then your Pokémon's Defense will raise faster than your other stats, since for every four Arons you KO, you get +1 to your Defense stat. Since the maximum EVs you can gain for one stat is 255, the EVs that you can use is actually 252, since 252 can be divided by 4. This is important if you go through a process of "EVs training," meaning that you will only battle certain Pokémon to get the EVs you want. Giving a Pokémon 255 EVs of one stat is a waste of 3 EVs, since instead of 255 + 255 = 510, you can split up 252 + 252 + 6 = 510, and the 6 EVs (actually 4) will be used for another stat. Two EVs will be left unused no matter what. Even if you're not so keen on undergoing complete EVs training, you can train in an area where there are many Pokémon that gives the EVs that you want. If you battle online, you will be able to set your Pokémon's EVs. This is very important because you can give up to 63 points to a single stat using EVs. I will now provide a list of which Pokémon receives what EVs.

Note: Although it seems better to try to look for Pokémon that gives +2 or +3 EVs, these Pokémon are probably very rare and are not populous, so it is actually faster to find an area with many Pokémon with +1 EVs and KO them instead.

Ways to speed up EVs Training
Yes there are, and they're not really hard to get (well some of them). The items or events below affect EVs Training in some way.

Macho Brace
The game does not tell you what the Macho Brace is for, besides the fact that it halves the wearer's Speed. That can't be all that it does, since it will make the Macho Brace useless. In fact, the wearer will gain double whatever EVs it gets. Fainting a Taillow while wearing a Macho Brace will give you 2 Speed EVs (Taillow originally gives 1 Speed EV). If your Pokémon is at a high level and won't be affected by the huge Speed decrease, then please use this great item!

EXP. SHARE
As you all know, the wearer of EXP. SHARE gets half of the experience while not in battle. Also, the wearer will get EVs, not half, but full. If your Jolteon faints a Taillow and your Zigzagoon (that was never in battle) has EXP. SHARE attached, then both Jolteon and Taillow gains 1 EV. This is a good way to train up weak Pokémon both experience-wise and EV-wise.

The PokéRUS
If you haven't heard of this before, the PokéRUS doubles the EVs received, just like the Macho Brace. This is good, but the PokéRUS is random whether or not it will infect one of your Pokémon. If you're lucky enough to have it, then it will infect your whole party. The PokéRUS will go away, so if you want to keep it so that it could infect new Pokémon, then put one infected Pokémon in a box, since the PokéRUS doesn't go away if the infected Pokémon is in a box. When the want to infect a new Pokémon, withdraw the infected Pokémon and when it infects your party, put it in a box again. Note that a Pokémon that has been infected by the PokéRUS will not get PokéRUS again, since its Memory T Cells will ensure that.