Several people have asked me what are Effort Values or EVs for short so I thought I'd address it in this post.
An EV, or Effort Value, is a certain value that helps to determine the growth of your Pokemon's stats. Every time your Pokemon gains experience from a battle, EVs will be given to certain stats depending on its opponents. There are 6 types of EVs, one for each stat: HP, Attack, Defense, Special Attack, Special Defense, and Speed; different Pokemon will provide different EVs upon defeat. This means that along with level-ups gaining stats, the EVs acquired by your Pokemon will also influence how much the stat goes up.