Serebii and Bulbagarden are NOT the places to go to to understand these concepts. There's only one place for this: Smogon.

Anyway, since Smogon's down, I'll give you the basic breakdown.

IVs and DVs are the same thing. They're individual values that are randomly determined when the Pokemon first appears. For Advance, they come in a set of 6: one each for HP, Attack, Defense, Speed, Special Attack, and Special Defense. The values can be from 0 to 31; with one exception, you want your IVs to be as high as possible, as these will make your stats higher. (The one exception is for getting a useful Hidden Power; as far as in-game, unless you're sharking, don't even bother with this. For Netbattle it's a little different.)

EVs, or effort values are a bit more complex. When you defeat an enemy Pokemon, whether wild or a trainer's, your Pokemon gets EVs. EVs, like IVs, are grouped by stat - HP/At/Df/Speed/SAt/SDf - and the more EVs, the higher the stat. Which EVs you get are dependant upon the Pokemon you defeat; Metagross gives 3 Defense EVs, while Tentacool gives 1 Special Defense EV (I believe). If you want to know what gives what EVs, there's a list here. (incidentally, that link will also give you a decent overview of various concepts.)

The tricky part starts with the caps. First off, each stat has a cap. If you defeat, say, 255 Tentacool (255 special defense EVs), then defeat another one, you won't have 256; you'll still have 255. The 255 cap applies to all stats. Second, there's a TOTAL cap of 510 - in other words, if you have 80 HP EVs, 156 Defense EVs, 252 Speed EVs, and 22 Special Defense EVs, that's 510 - and no matter what Pokemon you defeat from then on, you will never get any more EVs. Certain berries in Emerald will permit you to remove 10 EVs from a stat; but if you're dead set on EV training, this shouldn't matter.

And finally... making a battling Pokemon or even a team is not an exact science. Smogon can only go so far in bridging the gap; at best, you can try to put together a standard team, or steal someone else's, or get a skilled battler to help you make one (I'm not a skilled battler) - but you eventually have to develop the skills necessary to compete. Fortunately, if your only objective is to beat trainers in XD, you can take heart in the fact that the AI can be exploited, as it will rarely switch; but if you want to go online, I recommend taking up Netbattle. There's no substitute for real experience, and at the moment, Netbattle is the closest thing you'll get to DP online until the latter is out and available.