Originally Posted by nintendo forums

Individual Values (IVs)

Q: What are IVs?
A: Individual or Diversification Values (IVs or DVs, two names for the same thing) are points that act as a Pokemon's "genes." Every Pokemon has a number between 0 and 31 for each of its stats (HP, Attack, Defense, Special Attack, Special Defense, and Speed) that is hidden from the person playing the game. Every single Pokemon has different IVs, even two Pokemon of the same species. They can never be changed.

Q: How do IVs influence stats?
A: Simply put, the better your IVs, the better your Pokemon's stats. Ever notice how some Pokemon just seem to have an easier time in battles than others? Maybe you raised one Pikachu in a game. Then, starting a new game and catching a new Pikachu, you felt it performed better. The explanation? IVs. Every Pokemon within a species is identical upon catching them besides their IVs. At level 100, an IV equates to 1 stat point added to that stat. So if you have 31s across the board, you will have a very powerful Pokemon.

Q: How do you determine IVs?
A: First of all, you need to check them out before you start training. If you don't, your EVs will influence your stats, making it impossible to determine IVs unless you kept track of your EVs. Anyway, what you do is this. Make sure your Pokemon is at least at level 20. Record all of their stats. If they were hatched from and egg, save the game, use about 15 Rare Candies on it, record the stats, then shut off the game without saving. If you don't have that many Rare Candies, you'll have to train them. Big pain, because you'll have to keep track of EVs as well.