EXP, or Experience, is something that every Pokemon has. After a Pokemon has been defeated, every Pokemon that contributed will receive an amount of experience points. The older games featured an “equality” standard, in which each Pokemon that was in the battle received the same amount of EXP. This was revised in the recent games, where level is now a factor. There really isn’t a better way to explain than by using an example. Let’s say you are trying to level up a level 1 Pokemon. You have a level 50 Pokemon, and you took down another Pokemon with it after switching your level 1 out the first turn. The level 50 Pokemon will receive less EXP, and the level 1 will get the bulk of the EXP.
I don’t have an idea to why GameFreak revised the EXP system, but it definitely helps with low-leveled Pokemon. Instead of switching out your weak Pokemon the first turn, use the EXP Share, known amongst almost all trainers. This gives any Pokemon holding it EXP in the same way I described above. On a side note, the Pokemon holding the EXP Share will gain the same EVs as all who fought. If you’ve played before, you will eventually discover that each Pokemon grows differently and is classified into an EXP group based on how fast it levels. That number is the amount of EXP the Pokemon will have at level 100.
The first group is known as Erratic, which means that at level 100, you will have 600,000 EXP points. This is actually a very uncommon group; the only Pokemon I know that falls into it is Lumineon. Pokemon that are Erratic have weak stats, but it’s quicker to level up for those pressed with time.
The next groups are Fast, Medium Fast, Medium Slow, and Fluctuating. The amounts for these are 800,000, 1,000,000, 1,059,860, 1,250,000, and 1,640,000. Pokemon that level up slower tend to have better stats, but are extremely hard to train from the beginning. For example, most legends are classified as Slow because of their overwhelming power. In extreme cases, you can level up so slow it makes you really frustrated. Seviper is one of these, but does not follow the rule in having better base stats. The reason for fluctuating is unknown, though. To ease the burden of leveling up, Rare Candies can be found throughout the game. It is a better strategy to use this when your Pokemon are closer to level 100. A Rare Candy provides all the EXP for one level. If you are at level 99, a Rare Candy will fill up the EXP bar to 100. Don’t use this when your Pokemon is close to leveling up; it will be a waste. Find high-level Pokemon to fight, such as the Elite Four, to make training easier, or just leave it in the Day Care.