Basically any give pokemon at a given level has exactly the same stats as every other identical pokemon (so every Charmander has exactly the same stats as every other pokemon) what separates them is that when a pokemon is created, each of it's six stats is assigned an invisible bonus called an IV (or Individual value) and that number can be anywhere from 0 - 31 meaning that that stat has 31 extra points when it gets to level 100. IVs are generated when the pokemon is created (encountered or hatched) and cannot be changed. EVs (Effort Values) are awarded when you fight specific pokemon in the wild (Bunnelby will give you 1 EV for Speed every time you fight one, Scatterbug give you 1 IV for Defense every time you fight one and so on) and for every 4 EVs you earn, you get an invisible point in that stat. Like IVs, you don't see the full effect of the points until Level 100. You can get 255 EVs within two stats for a maximum of 510 EVs total for a pokemon. To maximize the efficiency, you should only give 252 EVs within 2 stats and the remaining 6 EVs to a third stat just so every point goes somewhere. (although you can distribute them however you want) For your trouble of 252 EVs within two stats, you end up with an additional 63 points in that stat at level 100. So couple the EVs with IVs and a pokemon can have up to an additional 94 points in a given stat.
When people refer to the number next to the IV they are referring to the number of IVs that are at 31. (So if I have a 4IV pokemon that means that 4 of it's stats have maximum IVs) It is technically possible to have 6 IVs, but it's very unlikely. The most realistic one people aim for is 5 IV because when breeding, there is an item called a Destiny Knot which will pass 5 IVs down from either parent (2 from one, 3 from the other, or any variation thereof) so the reason why 5 IVs are the most realistic is because if you have parents with a few IVS each, it's possible to make (while holding the destiny know) those stats be passed down to the child, so a father with 2 IVs and another with 3 IVs can have a child that has all 5 of those IVs. The 6th tends to be the stat that is being lowered by nature, so people usually don't care about it.
The best way to keep track of EVs depends on hwo you're doing it. If by battling, the easiest is to write them down: Find out what pokemon that are appearing in the area you are battling in give what, only fight the pokemon that give you the EVs you want and run from the rest. Write down and keep track. If by super training, your job is MUCH easier, because unlike when battling, Super training doesn't allow you to go over the 252 EV limit. So if you want to max out two stats, jsut pick the ones you want to max, then keep super training in that stat and using the stat bags you get as a reward until it tells you that stat can't go any higher. Then go onto the next, max it out, then go to a third stat, get the final 6 EVs and you're done!
