Originally Posted by Judge Dredd
They increase the point total of which stat you are focusing. So if you were trying to train a fighting pokemon you would battle Patrat or Lillpup. Each of them gives off 1 Attack Ev.
4 Evs =1 Stat Point
They can give your pokemon an advantage in battle.
Every pokemon you knock out in game gives at least 1 EV. Also vitamins (protien, iron ect) give 10 EVs for a single stat.