Effort Value, referred to by many players as EV, is a special kind of points aquired during battle that affect the overall stats of a Pokemon. These are incapable of harming a Pokemon, but when not properly utilized a Pokemon will not be at its maximum capability. The maximum number of points capable of being spread among all stats is 510, and only 255 can be put into any 1 given stat. For every 4 EV's 1 stat point is given. So, for example, if a Pokemon is given 40 EVs in attack it will now have 10 more points added to its attack stat. As you can probably tell by now EV's can be decently important and EV training a Pokemon properly can be very useful.
There are different things that affect EV's. The easiest to use are the vitamins, each one you give a Pokemon gives 10 into whatever stat it says it adds to, up to the maximum of 100. For example, Protein gives 10 to attack. Different Pokemon you battle will give stats, as shown in the list below. Remember, Pokemon only receive EV's when they receive EXP so in things like the battle tower no EV's are rewarded. The Macho Brace, Power Band, Power Lens, Power Bracer, Power Belt, Power Weight, Power Anklet, all increase the amount of EV's a Pokemon is awarded after a battle; however' while a Pokemon is holding this item its speed will be halved. This halving does not affect the permanent stats, and is removed when the Pokemon stops holding the item. A rare thing called the PKRS or Poke virus is a beneficial virus which doubles the EV's a Pokemon receives after a battle. Power Weight increases HP, Power Bracer increases Attack, Power Belt increases Defense, Power Lens increases Special Attack, Power Band increases Special Defense, Power Anklet increases Speed. Each of the Power items increases the EV's of whatever stat they affect by 4 per battle. The Macho brace doubles the EV's awarded at the end of battle.
At this point you must be slightly afraid of battling the wrong thing and ruining your Pokemon, which could mean having to re-breed to get another one with the Hidden Power and IV's you need. (If you have yet to figure out how these work please see the Hidden Power Thread and the Individual Value thread) The good news is no, you won't. You see, Emerald introduced a group of berries which state that they raise the friendship of a Pokemon but lower a base stat. This is a bit of a lie; they do increase friendship, however, it does not lower the base stats of a Pokemon. Instead, it lowers the EV's. The first berry you give will reduce the Ev's of the corresponding stat to 100, if it is over 100, and from there on it will reduce the EV's of that stat by 10. Using these berries you can retrain at any time as long as you have these berries. The berries are Pomeg Berry, Kelpsy Berry, Qualot Berry, Hondew Berry, Grepa Berry, and Tamato Berry.
Now you may feel ready to go. You have the Pokemon you want to EV train. You have the Macho Brace or the Power Item you want to make it go by quicker. You've decided where you want the stats to go. Now comes the fun of actually doing the training, and, simply put, to EV train you will just constantly battle the same Pokemon over and over until you have the number of EV's that you wanted into a stat. Yes, it can be tedious and, yes, it can be annoying, but it will pay off. So here is a list of Pokemon easily found through Pokemon Diamond and Pearl and the EV's they provide. I will not give a list of every Pokemon and the EV's the provide if you want that then please refer to www.serebii.net .
While There are more, this is a list of the ones that can be easily found throughout both Diamond and Pearl, that give only EV's to their respective stats. I hope that this guide helps you in your quest for the best. And when you have finished reading the entire list please vote in the poll or leave a message to help us upgrade the guide.
(Editted by GeneralBlueRaven for clarity.)