Effort Values.

I am kind of confused about the Effort Values. I never got to fully understand them. I heard it is something that helps your Pokémon grow stronger from each battle they do, however, they would earn less Effort Values under certain circumstances.

Let's say, there are 2 people that have the same Pokémon: A Rattata, for example. They would level it up to level 100, and evolving it too.

But, they would do it 2 different ways: The first person would grind for experience the long and hard way, by doing random battles; While the second person would just farm for Audinos, Chanseys and Blisseys.

Which of the two would have bigger Effort Values? Thanks in advance!

All Pokemon can have a maximum of 510 EV's, and 255 for each stat. So they would have the same. Audino, Chansey and Blissey are health/defense yielding Pokemon, so the Rattata which battled them would generally have higher stats HP and Defense wise. Every four EV's provides 1 stat of the type yielded. So you don't see a huge difference until higher levels. That's why I don't use rare candies.
Of course the stats defensively can only reach the species maximum, so the first Rattata is better of, because it has a higher chance of having more stat points over all. Once the second Rattata reaches maximum for Defense HP, any EV earned after that is wasted.
To avoid this there are items in Black White and newer version to remove EV's from a Pokemon.

Alright, awesome! Thank you for the explanation, Ice Dragon.