Pokémon Stratagem

EV's
Each Pokemon has six stats - HP, Attack, Defense, Speed, Special Attack and Special Defense. Effort Values are like points - each time you battle a wild Pokemon or an in-game trainer's Pokemon, it will increase your stat's EV count by a certain amount of points, for example, beating a wild Gardevoir will increase your Special Attack EVs by 3. For every 4 EVs you earn, the actual stat value raises by one (note this applies to Pokemon at lv100 only - at any other level, you may not notice an increase until more EVs are acquired). In other words, take the number of EVs, divide it by 4, and round it down to the nearest whole number. This gives you the stat increase.

Each stat can have a maximum of 255 EVs. So that means if you were to fight a wild Gardevoir 100 times, you'll only have 255 EVs instead of 300 (100 x 3). A Pokemon with 255 EVs for Defense (Or 254, 253, 252 since they are divided by four and round down to the same number) will have 63 points more than a Pokemon with 0 EVs for Defense.

However, a Pokemon can only hold a maximum of 510 EVs. That is, the sum of the EVs for HP, Attack, Defense, Speed, Special Attack and Special Defense must not exceed 510. Therefore, it's possible to max out two of a Pokemon's stats, whether it be Attack and Defense, or HP and Speed, or whatever. Careful planning means that you can have 252 for one, 252 for another, and 6 in another, meaning you max out two stats, and increase the last by 1.