Effort values

Effort values, or EVs, are a big part of Pokémon training, as much as levelling up. Effort values are experience points for Pokémon stats. Each Pokémon gives a different amount of effort points to each stat, and the more effort points you get in a certain stat, the higher the EV, raising that stat.

For every 4 effort points, you gain 1 more point upon leveling up than you normally would. There is a limit to EV gains introduced in Generation III, to prevent over-boosting your stats. You can gain a maximum of 510 EVs in total, and the most you can gain in a single stat is 255. This allows for the "maxing out" of 2 stats, which really means that you can gain a maximum of 63 points (~255/4) in one stat. EV training doesn't have to stop upon reaching level 100, at least in pre-Generation III; you can use the box trick. Their stats will be recalculated to reflect the gain in EVs.

Most Pokémon give only one EV in a particular stat, but most of the stage 1 and 2 Pokémon and Legendary/Rare Pokemon give up to 3 effort points in one stat, or multiple points for multiple stats..

Vitamins add 10 effort points to a particular stat, provided the EV is less than 100.

Macho Brace, an equip-item in Generation III, doubles the amount of effort points you gain from a Pokemon. So if you were battling a Rattata, which gives one Speed effort point, upon winning the battle you would recieve 2 Speed effort points instead of 1.

Exp. Share, other than sharing Exp. Points, also gives the same amount of EV's to the Pokemon holding it that the Pokemon battling recieves.

The rare disease Pokérus also gives double EVs to Pokémon afflicted as well as Pokémon that have developed resistance to it.