Every four effort values a Pokémon collects will contribute to one extra point stat on the next level up. Vitamin supplements like Carbos, Protein, and Iron will give ten effort values for their respective stats when given to a Pokémon. However, once the value of a stat is at least 100, the vitamin of that stat will no longer have an effect on that Pokémon. So for instance, if you EV trained an Eevee in Speed until there were 100 Speed EVs, a Carbos would become useless on that Eevee. Even with vitamin supplements, however, the stat changes will only take effect after a level up.
But even after the use of a vitamin has been barred, that EV can still be trained till the Pokémon has up to 255 EVs. Also, power items such as the Power Weight or Power Bracer will supply an additional four EVs every time EVs are gained in that particular stat, whether it be by vitamin supplement or defeating a Pokémon. So, if your Pikachu was holding a Power Bracer and defeated a Bibarel (which yields two Attack EVs), it would gain two attack EVs, plus four from the Bracer, making the total six.
A Macho Brace being held will work on any stat you are EVing, however it only doubles the amount of EVs earned, rather than adding four. It is best to hold it when you are taking a vitamin. :> A Pokémon with the Pokérus will earn double EVs every time it gains EVs as well. So if your Pidgey had the Pokérus and was holding a Power Belt, and you gave it an Iron, it would gain
10+4•2
A whopping 28 Effort Values. :>
After level up, that Pidgey would in turn gain
28÷4
seven extra points in the Defense stat, assuming it didn't gain additional Defense EVs from another source other than the aforementioned...
Also note that stat increases occur without EVs as well. Things such as IVs and natures cause the stats to naturally increase.
Anyway, I was just breeding Eevee for their natures and this one had a nature beneficial for Jolteon. So I made her a Jolteon! And "Bronti" is Greek for "thunder". ^^
Also, you can always check what EVs a Pokémon yields upon defeat.
And I should let you know that a Pokémon earns all of the EVs an opponent yields just by participating in the battle or holding an Exp. Share. :D