A much more complicated dynamic value are Individual Values, or IVs. Unlike its
counterpart (EVs), these are determined the moment you get your Pokemon, and
are pretty much what differentiates two Pokemon of the same species, and even
of the same gender, ability and nature.
IVs are six different values determined for the six main stats - HP, Attack,
Defense, Special Attack, Special Defense and Speed. They range from 0 to 31,
and the higher they are, the higher the base and maximum number a Pokemon's
stat may be.
Granted that you should aim for a Pokemon with an IV of 31 for all six stats,
but that might not be a case for several Pokemon. IVs are also the values
that determine the type and power of a move called Hidden Power. I do not know
how exactly you determine this, so look in another guide.
One other thing is that it is very, VERY unlikely for your Pokemon to have an
IV of 31 for all stats, let alone have an IV of 31 for one stat (and even if it
did, it may not be your desired stat), so it is extremely hard to breed for the
perfect IVs. Several IV breeders take months to get ONE perfect Pokemon, so it
is really dependent on luck.
There are two ways to influence the IVs of a Pokemon, and both of them involve
breeding, although one of them is merely a subproduct of the other, and was
introduced in HG/SS.
Whenever you breed a Pokemon, it inherits three of the twelve IVs from both
parents.
A bonus in HG/SS utilizes the six Power Items [Power Anklet, Power Band, Power
Belt, Power Bracer, Power Lens and Power Weight]. If either parent holds any
of these six items, their offspring will inherit a certain IV of their parent,
dependent on the Power Item held. To know which stat is inherited, refer to
the table with the Power Items on Section 3.3. The stat they affect corresponds
to the stat inherited by the offspring.
This means that you can ensure 2 IVs of the offspring, assuming that you know
the IVs of both parents. Both parents holding a Power item, though, means that
one cannot use the Everstone bonus to influence the offspring's ability, so
choose wisely.
To know the IVs of your Pokemon, it is best to resort to an online IV
Calculator. Serebii has one, but I'm pretty sure many sites have their own, so
pick one of your preference. IVs become more accurate if the Pokemon's level is
higher, so level-up your Pokemon should you want it exact. Should you only want
a range, 20-30 is already a pretty good level to find that out.
Oh, and as an extra bonus - introduced in Diamond and Pearl is a value called a
Characteristic. This tells you what the highest IV of a Pokemon is, which is
really helpful and a quick way to tell if your Pokemon has good IVs or not. It
does not give you exact values, though, so you should still resort to IV
calculators for that.
Here are the characteristics and the possible values that they imply. Note that
whatever value they imply is the highest value, so a single-digit IV could be
your highest one, which means you're really unlucky.
Loves to eat      || 0 | 5 | 10 | 15 | 20 | 25 | 30 |
Often dozes off   || 1 | 6 | 11 | 16 | 21 | 26 | 31 |
Scatters things often || 2 | 7 | 12 | 17 | 22 | 27 |     This one for
Often scatters things || 3 | 8 | 13 | 18 | 23 | 28 |             HP
Likes to relax     || 4 | 9 | 14 | 19 | 24 | 29 |
Proud of its power   || 0 | 5 | 10 | 15 | 20 | 25 | 30 |
Likes to thrash about || 1 | 6 | 11 | 16 | 21 | 26 | 31 |
A little quick tempered|| 2 | 7 | 12 | 17 | 22 | 27 |     This one for
likes to fight|| 3 | 8 | 13 | 18 | 23 | 28 |                 Attack
Quick tempered     || 4 | 9 | 14 | 19 | 24 | 29 |
Sturdy body       || 0 | 5 | 10 | 15 | 20 | 25 | 30 |
Capable of taking hits || 1 | 6 | 11 | 16 | 21 | 26 | 31 |
Highly persistent   || 2 | 7 | 12 | 17 | 22 | 27 |         This one for
Good endurance     || 3 | 8 | 13 | 18 | 23 | 28 |           Defense
Good perseverance   || 4 | 9 | 14 | 19 | 24 | 29 |
Highly curious     || 0 | 5 | 10 | 15 | 20 | 25 | 30 |
Mischievous      || 1 | 6 | 11 | 16 | 21 | 26 | 31 |
Thoroughly cunning  || 2 | 7 | 12 | 17 | 22 | 27 |     This one for SP.Attacks
Often lost in thought || 3 | 8 | 13 | 18 | 23 | 28 |
Very finicky      || 4 | 9 | 14 | 19 | 24 | 29 |
Strong willed     || 0 | 5 | 10 | 15 | 20 | 25 | 30 |
Somewhat vain     || 1 | 6 | 11 | 16 | 21 | 26 | 31 |
Strongly defiant   || 2 | 7 | 12 | 17 | 22 | 27 |     This one for
Hates to lose     || 3 | 8 | 13 | 18 | 23 | 28 |     Special Defense
Somewhat stubborn   || 4 | 9 | 14 | 19 | 24 | 29 |
Likes to run      || 0 | 5 | 10 | 15 | 20 | 25 | 30 |
Alert to sounds    || 1 | 6 | 11 | 16 | 21 | 26 | 31 |
Impetuous and silly  || 2 | 7 | 12 | 17 | 22 | 27 |     This one for
Somewhat of a clown  || 3 | 8 | 13 | 18 | 23 | 28 |      Speed
Quick to flee     || 4 | 9 | 14 | 19 | 24 | 29 |