Effort values (abbreviated EVs and previously called Stat Exp) are attributes which give bonuses to a Pokémon's stats and improve differently depending which Pokémon they defeat. These bonuses, in the form of effort points, are gained in addition to bonuses gained by increasing level. A Pokémon which increases in level using a Rare Candy instead of battling does not gain any EVs, making it weaker than a Pokémon who increases in level normally.
Yes, I know that passage says nothing about your question but "Evs" were just a simple(not really) mechanic programmed into the game. Some group of people figured out the "EV" system and have named it Effort Values. They have named it that because of how you have to fight and faint pokemon to collect them thus putting in effort for the hard work to collect them. Now it has become one of the most important mechanic in competitive battling since then.
EDIT: Okay, the company has installed the formula into the game mechanics and someone(probably Smogon, Serebii or just another group of people) have found out by creating systems like Pokésav to alter stats and mess around with it until they found out. When you say? They probably figured it out in 3rd Gen. (so around 2002) that there were Evs. The word spread of this formula and people have scoured all games for the mechanics on it.