what exaclty is it? Oh and whats are some of the most commen pokemon people will pay fg for?

Ev training is fighting against certain pokemin to make a certain stat go up. You also have tofind a compatable Pokemon nature to go along with the ev training.

Not exactly. You have the gyst of it.

Each pokemon you kill's dominant stat by nature; e.g. Pikachu gives 2 speed ev's once you K.O. one. Each 4 ev's in a stat will give an extra point in that stat at level 100, so if you mass ev train at level 5, it's not going to show much difference. Natures simply boost or decrease a stat, like modest will add 10% special attack and -10% attack.