[color=magenta]Well, a Pokémon's nature will effect it's stats. In Emerald, it will also effect it's personality (what moves it will use) in one of the Battle Tents, and the Battle Palace at the Battle Frontier. Some natures don't effect Pokémon stats at all, but most will increase one Pokémon's personal max stat by 10%, while decreasing another personal max stat by 10%. For example, a Pokémon with a Modest Nature will have a Special Attack stat that is 10% higher than it normally would, while it will in return have an Attack stat that is 10% lower than it normally would. Here is a chart listing the effects of the different natures.
Adamant: Attack(+), Special Attack(-)
Bashful: No Effect.
Bold: Defense(+), Attack(-)
Brave: Attack(+), Speed(-)
Calm: Special Defense(+), Attack (-)
Careful: Special Defense(+), Special Attack (-)
Docile: No Effect.
Gentle: Special Defense(+), Defense (-)
Hardy: No Effect.
Hasty: Speed(+), Defense(-)
Impish: Defense(+), Special Attack(-)
Jolly: Speed(+), Special Attack(-)
Lax: Defense(+), Special Defense(-)
Lonely: Attack(+), Defense(-)
Mild: Special Attack(+), Defense(-)
Modest: Special Attack(+), Attack(-)
Naive: Speed(+), Special Defense(-)
Naughty: Attack(+), Special Defense(-)
Quiet: Special Attack(+), Speed(-)
Quirky: No Effect.
Rash: Special Attack(+), Special Defense(-)
Relaxed: Defense(+), Speed(-)
Sassy: Special Defense(+), Speed(-)
Serious: No Effect.
Timid: Speed(+), Attack(-)