This is from the infamous "Help Thread" I made before. Let it be forever remembered.

Natures: Increase a stat by 10% and decrease another stat by 10%.

Adamant- Increase Atk, Decrease SpAtk
Brave- Increase Atk, Decrease Spe
Lonely- Increase Atk, Decrease Def
Naughty- Increase Atk, Decrease SpDef

Bold- Increase Def, Decrease Atk
Impish- Increase Def, Decrease SpAtk
Relaxed- Increase Def, Decrease Spe
Lax- Increase Def, Decrease SpDef

Modest- Increase SpAtk, Decrease Atk
Quiet- Increase SpAtk, Decrease Spe
Mild- Increase SpAtk, Decrease Def
Rash- Increase SpAtk, Decrease SpDef

Calm- Increase Sp Def, Decrease Atk
Careful- Increase SpDef, Decrease SpAtk
Sassy- Increase SpDef, Decrease Spe
Gentle- Increase SpDef, Decrease Def

Timid- Increase Spe, Decrease Atk
Jolly- Increase Spe, Decrease SpAtk
Hasty- Increase Spe, Decrease Def
Naive- Increase Spe, Decrease SpDef

Bashful- neutral
Docile- neutral
Hardy- neutral
Quriky- neutral
Serious- neutral

Everstone Trick = Give a Female Poke'mon or a Ditto an everstone in the daycare. And it shall have a high chance of passing the poke'mon's nature. This trick doesn't work all the time.  I believe it has a 50% chance of passing on the nature. This is extremely useful for breeding correct natures. 

Effort Values (EV's) = Points that are gained when training a poke'mon. There is a total of 510 per poke'mon. 255 is the max in a stat. 252 is required to max out a stat. Only 2 stats can be maxed out. If you do the math correctly, 6 EV points are leftover when you max out 2 stats.

Tier Rankings: 

Uber: Banned poke'mon from Standard Competitions.
OverUsed (OU): Most commonly used poke'mon.
BorderLine (BL): Poke'mon whose power is shifted between two ranks.
UnderUsed (UU): Poke'mon that are not strong enough to be in OU.
NeverUsed (NU): Poke'mon that are not meant to be in battle too much.


Hp = Cowgirl South of Solaceon. She has 5 Bidoofs.
Bidoof = 1 ev in Hp
+4 with Power Wieght
x2 with pKrS
10 total

Attack = Super Rod at Lake Acuity. Gyrados & Seakings.
Both of them = 2 ev in attack
+4 with Power Bracer
x2 with pKrS
12 total

Defense = Ruin Maniac's Cave. Geodude's & Hippopotas.
Both of them = 1 ev in defense
+4 with Power Belt
x2 with pKrS
10 total

Special Attack = Old Chateau. Gastly's & Haunters & Gengars.
Gastly = 1 ev in special attack
Haunter = 2 ev in special attack
Gengar = 3 ev in special attack
+4 with Power Lens
x2 with pKrS
10, 12, and 14 total respectively

Special Defense = Anywhere with Tentacools & Tentacruels.
Tentacool = 1 ev in special defense 
Tentacruel = 2 ev in special defense
+4 with Power Band
x2 with pKrS
10 and 12 total respectively

Speed = Magikarp Fisherman West of Eterna. He has 6 Magikarps.
Magikarp = 1 ev in speed
+4 with Power Anklet
x2 with pKrS
10 total