I like EV training. I'll EV train after I finish the storyline so I can breed for natures and moves and make an awesome 5th gen team for Wifi. :D
EDIT: EV stands for Effort Values. Every Pokemon you defeat adds to a certain stat. Pidgey adds to 1 EV to Speed, Luxray adds 3 EVs to Attack.
You can have 512 EVs, and a max of 255 in any stat. So you can pretty much max two stats if you want to by training on only a certain kind of Pokemon.
That's why rare candy doesn't make them stronger -- because they don't gain EVs, just levels.
They gain the stats upon level up, so you can't EV train a level 100 Poke, but you can EV train a level 99 poke whos EVs you've lowered with EV reducing berries.