(To spideboy) IV's, and EV's aren't too hard to understand, they just look complicated on paper. This isn't exactly a 1 for 1 on how they work, but a simplified version to help you understand. Not sure if you wanted to know this stuff, but just in case. Hope it helps if so. This was a simplified version though, lots of maths involved in how it actually works (well in EV's atleast).
Let's say Pokemon A has 100 in every stat, so -
HP -100
Attack - 100
Special Attack - 100
Defense - 100
Special Defense - 100
Speed - 100
-------------------------------------------NATURE------------------------------------------------------
Nature simply gives you minus 10% to one stat, and plus 10% to another, so take Adamant nature, +Atk, -S Atk.
Hp -100
Attack - 110
Special Attack -90
Defense - 100
Special Defense - 100
Speed - 100
----------------------------------------------------------IV-----------------------------------------------------------
IV's are like DNA. Every Pokemon is born with a radom dice roll inbetween 1 to 31 in each stat. So let's say Pokemon A is born with these IV's -
Hp - 12
Atk - 21
S Atk - 3
Def - 14
S Def - 17
Spd - 7
Pokemon B is the same species, has that same Adamant nature, but has different IV's. -
HP - 22
Atk - 11
S Atk - 16
Def - 2
S Def - 28
Spd - 31
So Pokemon A, despite being the same species, same nature, is 24 points slower. It's not a huge difference, but if you play competitively it can make the difference inbetween a win and a loss.
------------------------------------------------EV----------------------------------------------
EV's are Effort Values. Think of them as the effort you put in at the gym to get stronger. You can have a total of 510 EV points per Pokemon, but only 252 in one stat. Each Pokemon you fight can give you 1-2 EV points in a specific stat. Like I think Wingull gives 1 Speed EV. EV's are not permanent as you can reset them. They make a much larger difference to stats than IV's however. I'm not going to get into the math of it as I don't completely understand it myself, but it gives it's full benefit at level 100, and scales down at lower levels.
Basically EV's are what let you customize your Pokemon's stats. You want to have a fast Pokemon, go fight 252 Wingull's (there are ways to greatly speed that up), and now your Pokemon is decently faster. Want it to be tanky? Put 252 EV's into HP, 252 into Defense, and the last 4 into Special Defense. EV's are probably the most difficult part to understand, but very important if you want to get the most out of your 'Mons. No matter how you try a Magikarp may never beat a Greninja in Speed, but maximizing it with a Speed + nature, 31 in Speed IV's, and 252 in Speed EV's makes a huge difference.