Natures are key to a pokemons stats and the moveset you choose for it. Each nature raises and hinders a stat, except for 5 natures. The nature you choose is most likely the nature that raises the stat you need and hinders the stat you don't need. For example, Adamant you use if your pokemon is going to be physical and you don't need Special Attack. So here is a list of the natures.

Adamant +Attk -Sp.Atk

Brave +Attk -Speed

Naughty +Attk -Sp.Def

Lonely +Attk -Def

Relaxed +Def -Speed

Impish +Def -Sp.Atk

Bold +Def -Attk

Lax +Def -Sp.Def

Hasty +Speed -Def

Timid +Speed -Attk

Jolly +Speed -Sp.Atk

Naive +Speed -Sp.Def

Modest +Sp.Atk -Attk

Mild +Sp.Atk -Def

Quiet +Sp.Atk -Speed

Rash +Sp.Atk -Sp.Def

Calm +Sp.Def -Attk

Gentle +Sp.Def -Def

Sassy +SP.Def -Speed

Careful +Sp.Def -Sp.Atk

Quirky -------- ---------
Bashful -------- ---------
Docile -------- ---------
Hardy -------- ---------
Serious -------- ---------

So there it is........so yeah....straight forward....