Effort Values - The Basics
Effort Values (EVs for short) are, simply put, skills that a Pokémon trains for in order to increase some of their stats. Unlike Individual Values, EVs are directly manipulable by the player.
Imagine, for example, a basketball player who has born with a talent for 3-point shots but is not a very good defender. When they are practicing, they could choose to either improve on their 3-point shots and become even better at something they are already proficient in, or they could practice on their defense to shore up a weakness in their skills.
This is essentially what EVs are for Pokémon. You could choose to increase a Pokémon's speed by training them in a way that gives them more Speed EVs, or give them more HP by letting them get more HP EVs. They let you customize your Pokémon to either improve on what they are already proficient in or balance them out by covering for a weakness. This allows for greater variety in Pokémon, as you can choose which EVs to give your Pokémon.
Imagine, for example, a basketball player who has born with a talent for 3-point shots but is not a very good defender. When they are practicing, they could choose to either improve on their 3-point shots and become even better at something they are already proficient in, or they could practice on their defense to shore up a weakness in their skills.
This is essentially what EVs are for Pokémon. You could choose to increase a Pokémon's speed by training them in a way that gives them more Speed EVs, or give them more HP by letting them get more HP EVs. They let you customize your Pokémon to either improve on what they are already proficient in or balance them out by covering for a weakness. This allows for greater variety in Pokémon, as you can choose which EVs to give your Pokémon.
Acquiring EVs and How They Affect Your Pokémon
In the game, EVs are acquired by defeating Pokémon, both wild and trainer-owned. Certain species of Pokémon give out certain EVs when they are defeated. For example, Zubat (and generally other speedy Pokémon) give out Speed EVs whenever you defeat them. As such, if you want to make your own Pokémon faster, you would have to focus on defeating things like Zubat until you get the amount of EVs you want.
Limits are placed, however, to keep you from simply giving a Pokémon a near infinite amount of EVs for every stat. Each Pokémon can only get a grand total of 510 EVs, and each of the 6 specific stats (HP, Attack, Defense, Special Attack, Special Defense, Speed) are only allowed a maximum of 255 EVs each (252 in Pokémon X and Y). This means that you are limited in the amount of EVs you can give your Pokémon, and you cannot dump all of them into one slot.
Furthermore, a common misconception among newcomers is that EVs are directly tied to actual stats in a 1:1 ratio. However, pumping 252 Speed EVs into your Charizard does not mean that he will magically gain an extra 252 points to his Speed stat. Rather, there is a complicated formula that decides how much the stat is affected by the amount of EVs it has. At level 100, for example, every 4 EVs equates to 1 extra stat point. For the Charizard example above, the 252 Speed EVs at level 100 translates to 63 extra points to his Speed stat (252 / 4 = 63).
This means that to make the most use out of them, the amount of EVs you should give your Pokémon should always be divisible by 4. For example, if you give your Charizard 4 EVs in Speed and you friend gives theirs 5 EVs, at level 100, they will both only get one extra stat point and thus be functionally the same (speed-wise).
Limits are placed, however, to keep you from simply giving a Pokémon a near infinite amount of EVs for every stat. Each Pokémon can only get a grand total of 510 EVs, and each of the 6 specific stats (HP, Attack, Defense, Special Attack, Special Defense, Speed) are only allowed a maximum of 255 EVs each (252 in Pokémon X and Y). This means that you are limited in the amount of EVs you can give your Pokémon, and you cannot dump all of them into one slot.
Furthermore, a common misconception among newcomers is that EVs are directly tied to actual stats in a 1:1 ratio. However, pumping 252 Speed EVs into your Charizard does not mean that he will magically gain an extra 252 points to his Speed stat. Rather, there is a complicated formula that decides how much the stat is affected by the amount of EVs it has. At level 100, for example, every 4 EVs equates to 1 extra stat point. For the Charizard example above, the 252 Speed EVs at level 100 translates to 63 extra points to his Speed stat (252 / 4 = 63).
This means that to make the most use out of them, the amount of EVs you should give your Pokémon should always be divisible by 4. For example, if you give your Charizard 4 EVs in Speed and you friend gives theirs 5 EVs, at level 100, they will both only get one extra stat point and thus be functionally the same (speed-wise).
The Importance of EVs
63 extra stat points for 252 EVs does not seem like it would be a big deal, that it wouldn't really heavily affect a Pokémon's strengths and weaknesses. However, they are extremely important. Let's take a look at an example. Say you have a Level 100 Charizard, fighting a Level 100 Blastoise. In all these battle situations, we will assume the weather is Sunny (fire-type attacks strengthened, water-type attacks weakened).
According to Smogon's Battle Calculator, Charizard's Solarbeam will only take 78.2 - 92.3% of Blastoise's HP away, thanks to his increased Special Defense. The amount of HP Charizard takes away will guarantee that Blastoise's Torrent kicks in (Water-type attacks become stronger when user HP is low), so when Blastoise retaliates with Hydro Pump, Charizard will lose 115.8 - 137.3% of his HP. Blastoise will have very low HP, but he will get a guaranteed One-Hit KO on Charizard.
Now, let us take a similar situation as above, with one key difference: Charizard will have 252 Special Attack EVs to further boost his power.
Now, let us take a similar situation as above, with one key difference: Charizard will have 252 Special Attack EVs to further boost his power.
In this situation, Charizard will take 97.6 - 115% of Blastoise's HP. While it isn't a guaranteed OHKO, it is very likely that Charizard will defeat Blastoise. By giving your Charizard 252 Special Attack EVs, you go from having no chance of defeating Blastoise to having a very good chance of coming out on top.
Granted, in most cases, EVs will not be the sole deciders of your victory or defeat, but they definitely have a huge effect on the flow of battle. Whether you decide to make your Pokémon better at something they're already very good at, or you cover for one of their weaknesses, EVs can help keep your Pokémon in battle. EVs are essential in competitive battle, and learning how to apply them will take you a long way in your quest to become a competitive Pokémon battler.
Granted, in most cases, EVs will not be the sole deciders of your victory or defeat, but they definitely have a huge effect on the flow of battle. Whether you decide to make your Pokémon better at something they're already very good at, or you cover for one of their weaknesses, EVs can help keep your Pokémon in battle. EVs are essential in competitive battle, and learning how to apply them will take you a long way in your quest to become a competitive Pokémon battler.
Further Reading
Click HERE for a more in-depth explanation of Effort Values on Bulbapedia, including other ways to manipulate EVs and the history of Effort Values throughout the generations of Pokémon.