One of the additions to the pokemon games, when Ruby and Sapphire came out, was the introduction of natures. Natures introduce an additional level of complexity to a pokemon's development and help to reinforce certain trends in the pokemon metagame.
In addition to natures, with the release of Diamond and Pearl, characteristics were also introduced which are basically another strata of natures, allowing pokemon to further lean toward developing in a certain way. However, natures remain the dominant force that we trainers should be interested in when breeding/training pokemon.
And, if you didn't already know, nature determine how stats grow upon leveling up.
Natures automatically fall into two different categories: Neutral and Determinant. Determinant natures will yield a 10% increase in one stat and a 10% decrease in another while Neutral natures yield no stat changes whatsoever.
Neutral Natures include Serious, Hardy, Quirky, Bashful, and Docile.
Takes naps a lot
Daydreams a lot
Loves to Relax
Proud of its strength
Loves to thrash around
Loves to fight
Hates to lose
Loves to race
Sensitive to sound
Runs away fast