Natures are very helpful in raising your pokemon. They raise one statistic by
1.1x and lower another one by .9x. The nature determines which stats get raised
and which get lowered. You want to be sure that the pokemon you are raising
has the correct nature for what you want it to be. The following natures do not
affect stats: Hardy, Docile, Bashful, Serious and Quirky. You may want a
pokemon like this but this is usually not the case. You should once again pick
a nature that is beneficial to what you are raising. Adamant is a great nature
for a physical pokemon because it does not affect defense or speed and raises
physical attack. Below is a chart for natures that does not include the ones I
mentioned above:

NATURE.............UP......................DOWN
Lonely.............Attack..................Defense
Brave..............Attack..................Speed
Adamant............Attack..................Special Attack
Naughty............Attack..................Special Defense
Bold...............Defense.................Attack
Relaxed............Defense.................Speed
Impish.............Defense.................Special Attack
Lax................Defense.................Special Defense
Timid..............Speed...................Attack
Hasty..............Speed...................Defense
Jolly..............Speed...................Special Attack
Naive..............Speed...................Special Defense
Modest.............Special Attack..........Attack
Mild...............Special Attack..........Defense
Quiet..............Special Attack..........Speed
Rash...............Special Attack..........Special Defense
Calm...............Special Defense.........Attack
Gentle.............Special Defense.........Defense
Sassy..............Special Defense.........Speed
Careful............Special Defense.........Special Attack

If you are breeding and the mother pokemon has a nature you want then attach an
everstone to it. You then have a 50% chance of getting the same nature as the
parent.

Examples of good natured pokemon:
Alakazam    =  Modest
Dragonite   =  Adamant
Crobat      =  Jolly

Pick a nature that works for your moveset. Sometimes it is hard to get the
nature you want so do not pass up on a high IV pokemon unless the nature is
really bad. For example, I have an all-around 31 IV Alakazam that is Rash. That
works fine because it raises what I want and does not affect it's speed. Modest
would have been better but you take what you can get. Do not sacrifice nature
for IVs though. A perfect IV pokemon with a bad nature is usually worse than a
mediocre IV pokemon with a helpful nature