Wow. A whole lot of misinformation being bandied about in this thread. I would ask that if you are unsure about things, please don't post as it simply only adds confusion to the original question. As for the ORIGINAL question, I will do my best to provide as much information as possible:
Ok I need to know if there is a difference in the stats of a pokemon if you feed it rare candies instead of training them? Any help greatly appreciated.
There is a HUGE difference in the statistics of a Pokemon between increasing its levels by training (battles) or feeding it Rare Candies. Before I can address that though, there are a few things I need to make sure you undersand first -
1. Every Pokemon in the game has a preset value assigned to each stat (HP, Atk, Def, SAtk, SDef, Spd) referred to as an Individual Value (IV). This IV is a random number from 1-31 that is added directly to that particular stat. These values are what make each Pokemon in the game unique. For example, you can catch two Lv5 Taillow with the same Nature (using a neutral nature in this example), and one will have an Atk of 10 while the other has 12. While that doesn't seem significant, let's feed both Pokemon Rare Candies until they reach Lv100. At that level, the first Taillow has an Atk of 115, while the second Taillow has 146. We can see from our example that the first Taillow had an IV of 0 while the other had 31.
2. Every Pokemon in the game has a nature that can possibly increase the value of a particular stat by 10%, while decreasing the value of a different stat by 10%. For example, if the Taillow we used previously had an Adamant Nature, its Atk would be increased by 10%, while its SAtk would decrease by 10%. So, if our second Taillow had an Adamant Nature, it would now have an Atk of 160. A Pokemon's Nature is VERY important when determining whether or not it deserves a spot on your team, or if it will just take up space in your PC.
3. Every Pokemon in the game is able to earn Effort Value (EV) points. The points can ONLY be earned by battling wild Pokemon or AI battles in-game (including Game Cube games). Every Pokemon in the game also has an assigned EV point value in a particular stat. For instance, defeating a Raichu in battle will earn your Pokemon 3 Spd EVs while defeating a Clefable with earn 3 HP EVs. A Pokemon can earn a maximum of 510 EVs, including a limit of 255 EVs in a particular stat. For every 4 EV points earned in a particular stat, the stat in question will increase by 1. That means EV training can increase a particular stat by 32 points. For our second Taillow, that means at Lv100 its Atk will be raised further to a robust 229 (taking into consideration the 10% increase for the Adamant Nature).
The end result is you have one Lv100 Taillow with an Atk of 115, while another Taillow can have an Atk of 229, or 114 points higher.
But, what the hell does this have to do with the original question? Glad you asked. Feeding Rare Candies to a Pokemon will increase its level, but it will not gain any EVs that it would get through battling. On the flip side, leveling up your Pokemon through battling only is not only time consuming, but it can leave your Pokemon with EVs in undesired stats.
The best way to train your Pokemon is to first get a Pokemon with the desired Nature, Ability, IVs, and Gender (if applicable). The next step is to level it up by feeding it Rare Candies to at least Lv20. It is at this point you would begin EV training, only involving your Pokemon in battles that will give it EV points in the desired stats. Once the EV spread has been achieved, you can either feed the Pokemon Rare Candies until it reaches the level you want, or you can just level it up thorugh battling without having to worry about fixing its EVs later.
Don't forget the useful tools that are available to you in regards to EV training. First and foremost is the Macho Brace. Having your Pokemon hold this will double the EV points it earns in battle. For example, battling a Raichu will yield 6 Spd EVs instead of 3, effectively cutting your EV training in half. Next is the Exp Share. A Pokemon holding this will not only earn Exp Points in battle, but it will also earn the EVs that were accumulated as well. Don't forget the expensive vitamins in the game. Each one you feed to your Pokemon will increase the EVs in that particular stat by 10. Be aware though that vitamins will only increase EVs in a stat to 100. That means if you have done no prior training, you can feed a Pokemon 10 of them, but if you have already done training in a stat, you can only feed it until it reaches 100. In Emerald (and possibly Diamond/Pearl) there are certain berries that will actually reduce EVs in a particular stat by 10. This is useful to remove unwanted EVs in an undesired stat so that you may apply them in a different stat. Last and the best, not to mention the rarest, is Pokerus. Pokerus is a virus that your Pokemon can acquire by battling wild Pokemon. Once infected, it works exactly the same as the Macho Brace, doubling the EVs your Pokemon earns in battle. A side benefit is that when a Pokemon contracts the virus, for the first 48 hours, it can spread the virus to other Pokemon in your party during battle. Just make sure to deposit an infected Pokemon in your PC since there is no time in the PC which allows it to remain contagious for a much longer period of time.
That concludes Pokemon Training 101.
