Yeah.. better checking it somewhere 'trustful'...

But here's how I understand it works:

Every pokemon has a set of EVs which can be obtained defeating them. It's based moslty on it's highest stat and its stage.
Macho brace doubles the EVs gained
Power items (bracelet, belt) add 4 EVs to their corresponding stats. It doesn't matter if you get completely different EVs from the pokemon you defeated
Pokerus doubles the TOTAL gained in a battle

"If a Pokémon holds an Exp. Share, it will receive effort points even if the battling Pokémon has maxed out its effort points. If the Pokémon with the Exp. Share has Pokérus, the amount of effort points received is doubled."~*
Vitamins add 10 EVs in their corresponding stats, only up to 100. (i.e Def has 105, no more iron can be used)

"Pokémon are limited to a total of 255 effort points per stat, and 510 effort points in total. (...) only 252 effort points are required to maximize a stat."~*
That means you can maximize up to 2 stats.

Alright, let's put an example vs a Floatzel
Floatzel EV- 2
Macho Brace- *2
Pokérus- *2
TOTAL: 8 EVs
That means you must defeat 31 floatzels to get 248 EVs, then take the Macho brace off and get 4 more. Getting the 252 EVs

Another way (which I like the most)
Floatzel EV- 2
Power Anklet- +4
Pokérus- *2
TOTAL: 12
Only 21 floatzels need to be defeated.

Hope this helps!

Okay, take a deep breath here...

Every stat has EV points for it: HP, Attack, Defense, Sp. Attack, Sp. Defense, and Speed.

For every 4 points in a stat, that stat gets +1 points.

At max, a Pokemon can have a total of 512 EV points (this is when a person in game will say that your Pokemon has worked hard) and only 255 points in one stat.

BUT! 255 does not divide by 4 into an even number and does not round, so pushing a EV count to 255 is considered very bad practice. Instead, go only up to 252 (which divides evenly with 4) and save the other 3 for a different stat. Usually, people put 252 into two stats, then 6 into a third stat. Nothing can be done for 2 EV points.

Vitamins give +10 EV points for one stat, but only up to the 100th point. These are used to make EV training much easier.

Every Pokemon gives a certain amount of EV points, but only up to 3. They are not consistent, sometimes not making sense, and should be looked up instead. Recommended hotspots varies from specific game to game due to different Pokemon giving different stats.

There are also certain berries that will lower the EV points but increase happiness. For EV points over 100, they reset it to 100, then continue dropping it by 10. (Note that said berries can only be used if there is at least 1 EV point in said stat.)

Macho Brace will give Pokemon x2 the amount of EV Points they receive normally in battle. (The speed drop is only when holding the item and calculated for each turn in battle.)

Experience Share and participating in battle does not split the EV Points. Instead, if the Pokemon defeated gives 2 EV Points in Speed, then everyone in the battle place the Exp. Share users get 2 EV Points in Speed (not including any other item being used, which would also be calculated for the user only.)

There are also training items that are much like the Macho Brace in that they slow down, but instead give +4 for a certain stat (stated in description) after participating and winning a battle with a Pokemon as well as the original EV Points given from said Pokemon. So if a Pokemon holds one that increases HP and defeats a Pokemon that gives +2 in Speed, the Pokemon gets a total of +4 HP & +2 Speed for EV Points.

For 3rd and 4th generation, a level-up is required for the EV points gained to quick in. This makes it that level 100 Pokemon cannot be EV trained or fixed. So unless you got them right, they are stuck like that. 5th generation however fixes this and level 100 Pokemon have their stats adjusted to reflect any changes in EV points as well as being able to continue gaining them.

EV Points cannot be bred.

People do use EV Points for competitive play and the system adds to a layer that makes training a Pokemon for a long time makes them stronger while also punishing those who hack their game without knowing as to what they are doing. Also, they can make a big difference for the console games where you are the same level as your opponent all the time.

For this kinda stuff, I'd just say go to serebii or Marilland's site.

Better to get it from "smart pokemon players" than random "people on an internet forum".