From StrategyWiki, the free strategy guide and walkthrough wiki

First, please note that Natures only appear in the Third and Fourth Generations. Natures are like Traits; they are automatic. However, Natures raise a certain stat while lowering another stat, or they could have no stat changes at all. The change will be 10% in either direction. Also, all Pokémon can randomly have any Nature from the list below, while each Pokémon can only have one or two Traits assigned to them. Natures are like IVs in a way. If you want a Pokémon with a specific Nature, it is very hard to do so because it is random, but still possible since Natures are not hidden and there are fewer Natures than there are IVs. Natures are important because they give a boost in your Pokémon's strengths. If you are Machamp, you would love a 10% boost in Attack while would not mind a 10% decrease in Special Attack. Below is a list of Natures in the order of what it increases, what stat they increase, and what stat they decrease.

Nature	Bonus	Penalty
Bashful	No change	No change
Docile	No change	No change
Hardy	No change	No change
Serious	No change	No change
Adamant	+10% Attack	-10% Special Attack
Brave	+10% Attack	-10% Speed
Lonely	+10% Attack	-10% Defense
Naughty	+10% Attack	-10% Special Defense
Bold	+10% Defense	-10% Attack
Impish	+10% Defense	-10% Special Attack
Lax	+10% Defense	-10% Special Defense
Relaxed	+10% Defense	-10% Speed
Mild	+10% Special Attack	-10% Defense
Modest	+10% Special Attack	-10% Attack
Quiet	+10% Special Attack	-10% Speed
Rash	+10% Special Attack	-10% Special Defense
Calm	+10% Special Defense	-10% Attack
Careful	+10% Special Defense	-10% Special Attack
Gentle	+10% Special Defense	-10% Defense
Sassy	+10% Special Defense	-10% Speed
Hasty	+10% Speed	-10% Defense
Jolly	+10% Speed	-10% Special Attack
Naïve	+10% Speed	-10% Special Defense
Timid	+10% Speed	-10% Attack

Note that Natures do not alter a Pokémon's HP stat.