Effort values, also known as EVs, are values that determine the stats of your Pokemon. They are gained every time your Pokemon gain experience in battle. Each Pokemon gives a certain number of EVs in certain stats. Typically, Pokemon give 1, 2, and/or 3 EVs in one or two of the 6 stats of HP, Attack, Defense, Sp. atk, Sp. def, and Speed. For example, KOing a Starly gives 1 EV in speed, KOing a Staravia gives 2 EVs in Speed, KOing a Machop gives 1 EV in Attack, etc, etc. Having 4 EVs in a stat raises that stat 1 point higher at lv. 100 than it would be without it. For example:
Jigglypuff with 0 HP EVs has 330 HP at lv. 100
Jigglypuff with 4 HP EVs has 331 HP at lv. 100
Any given Pokemon can have a maximum of 255 EVs in a single stat and 510 total. In this way, you can use your EVs to max out two of the six stats that you choose. A potential (255/4 =) 63 point difference in a stat is not something to be ignored.
EV training is the process of KOing certain Pokemon over and over again to max out your EVs. For example, if you wanted to max out your attack and speed, you'd KO 255 Machops and 255 Starly. However, KOing that many Pokemon is quite tedious. Here are some tips to EV training:
Vitamins: Using the items like HP Up or Protein gives 10 EVs in the stat in the stat it says it raises (as long as that stat doesn't already have 100 or more EVs already).
Macho Brace: This doubles the amount of EVs you gain during battle. While holding this, instead of 255 Machop to get 255 EVs in attack, you now only have to KO (255/2 =) 128 Machop to max out your attack.
Power Items: These items are sold at the Battle Tower/Frontier for 16 BP each. This adds 4 EVs in the stat it says it raises. For example, holding the Power Anklet (Speed) while KOing a Machop will result in gaining 1 Attack EV and 4 Speed EVs. KOing a Starly results in (1 + 4 =) 5 Speed EVs. In this way, if you were to only battle Starly, instead of 255 Starly, you'd only have to KO (255/5 =) 51 Starly.
Pokerus: This is a rare status condition that doubles the amount of EVs you get in a battle. The benefit with this is that you can have this status condition and still hold an item at the same time. Alone, it works just like the Macho Brace. If your Pokemon has Pokerus and then holds a Macho Brace, the amount of EVs it gains in battle will be quadrupled. Now instead of having to KO 255 Machop to get 255 EVs in attack, you now only have to KO (255/4) = 64 Machop to max out your attack. Same deal with the Power items; if you were to have a Pokemon with Pokerus hold a Power Anklet (Speed) and then KO a Starly, it would gain a respectable (2 x [1 + 4] = 2 x 5 =) 10 EVs in speed. In this way, you'd only have to KO (255/10 =) 26 Pokemon to get 255 EVs in speed.
Does this make sense or did I completely lose you? ^^;
If I did, feel free to ask about what you don't understand.
