EV Training

EV stands for Effort Values.
EVs determine what stats your Pokemon will gain when it levels up. Certain Pokemon you battle, whether it's a trainer or a wild Pokemon, will raise the stats according to their stats, their nature, and your nature. Look in your party/pc box and check out some of the natures your Pokemon have.

Any Pokemon that likes bitter food will have higher special defense.[gentle, sassy, careful, calm]
Any poke that likes dry food will have higher sp. Attk [rash, mild, quiet, modest]
Any poke that likes sour food will have high defense [bold, lax, impish, relaxed]
Any poke that likes spicy food will have higher attack [adamant, naughty, lonely, brave]
Any poke that likes sweet food will have high speed [jolly, hasty, naive, timid]

If it says "happily eats anything" then that means all the stats will be evened out, a little bit of all the natures. [quirky, bashful, hardy, serious]

Each Pokemon you fight has a certain base stat that is signifigantly higher than the other base stats. For example, if you battle a starly and level up, you will gain higher speed points for your stat that raises.

For each 4 EVs that you gain for one stat in one level, you will gain 1 stat point up when you level up. If you have 10 EVs for one stat, you will gain 2 stat points up when you level up. If you have 20 EVs for that stat, you will gain 5 Stat points up.

Your max stats at level 100 can not excede 510 EVs. The max one stat can go for EVs is 255, but you only use 252 of that.

Here are some Pokemon that you can level up according to base stats:
HP STAT= Bidoof (+1) Gastrodon (+2)
SPEED STAT= starly(+1 EV) staravia (+2 EV) buneary (+1)
ATTACK STAT= staraptor(+3) Kricketune (+2) Luxio (+2)
Sp ATTK STAT= budew (+1) girafarig (+2)
Sp DEF STAT= Misdreavus (+1) Togekiss (+1/+2 attack stat also)
DEFENSE STAT= hippopotas (+2) Skarmory (+2)

Each Pokemon has a higher stat. These are only a few. All 493 Pokemon there are to battle(even if you use poke-modifier codes) give EVs when you are levelling up. EVs add up when you are gaining EXP points, so the more Pokemon you battle, the more EVs you'll gain, and the more powerful your Pokemon will become!

Http://www.youtube.com/watch?v=EIUPhmaklOI
There is a link to a video that may help you. Hope I helped