For every 4 EVs you gain, you get +1 in that stat at lv. 100. Since the max is 255 EVs in a stat (recommended max is 252; total max for all EVs is 510), you can only have that stat be 63 points higher than if it wasn't EV trained. Keep in mind that these stat bonuses are distributed evenly among the 100 levels. Unless you start EV training (with an untouched Pokemon) at a really high lv., you aren't going to notice much of a difference in the stats gains.
For example, I EV train a Blissey with all 252 in HP and finish at lv. 15. Normally, without EVs, Blissey's HP gain would be like +3 or +4. However, after it levels up to with finished EV training, it may have a boost like +7 or +8. Afterwards the stat boosts would mostly be +4 with some +5s thrown in. Not really a noticeable difference.
This was a concept I found hard to grasp at first, so...