Every pokemon has two hidden numbers in their data, called Individual Value and Personality Value. They are random for every wild pokemon you encounter and you can't change them. They determine a whole range of attributes including stats, gender, ability, nature and characteristics.
Individual Values (IVs for short) are the key to determining your pokemon's stats. IVs are randomly assigned when you encounter any wild pokemon (or receive it as a gift or egg). They consist of six numbers corresponding to HP, Attack, Defense, Special Attack, Special Defense and Speed. Each range from 0-31 and the higher the number, the better your pokemon's stat will be.
IVs also determine the type and strength of the move Hidden Power. In Pokemon Platinum, HeartGold and SoulSilver, you can find out what type Hidden Power would be for any pokemon by showing it to a man in the game corner Prize Exchange building (Veilstone City for Platinum, Celadon City for HG/SS).
Personality Values were introduced in Generation III. Like IVs they are a random number assigned to the pokemon upon encountering or receiving it. They determine most of a pokemon's attributes:
Characteristics
are another in-game barometer of key strengths. In your pokemon's summary
it will say one of the following messages, which tells you your pokemon's best IV stat. For
example, if it says Proud of its power
then its Attack IV is higher than the other IVs.