What is effort value in Pokemon?

Effort Value is an internal stat list inside the Pokemon besides the obvious stats.

Depending on how your EVs are, it will affect the amount of points whatever stat is being 'EV Trained' gets.

Say that you EV train a Pokemon's Sp. attack, when he levels, he'll get more than just 0, 1, or 2 points.

Sometimes as good as 7, depending on how well you were doing it.