what is an "ev"?
EV stands for “Effort Value.” They determine what stats your Pokémon gets upon levelling up and are gained from battling any Pokémon you come across that award Experience points. For example;
Beating a Caterpie would give the Pokémon used in battle +1 Effort Value to HP
Beating a Tauros would give the Pokémon used in battle +1 Effort Value to ATTACK and +1 to SPEED
How do EVS AFFECT STATS?
For every 10 EVs you gain in a specific stat, you'll gain +1 to your Pokemon's base value in that stat. What this basically means is that if you battle 10 Caterpie with your Shuckle, Shuckle will then gain +1 to its HP stat (because each Caterpie gives +1 EV in the HP stat).
EVs are calculated at +1 Stat point per 10 EVs at level 100, which means you won't see the full benefit of the EVs you put into your Pokémon until it reaches that level; there will still be a benefit, just not as significant as when it reaches the maximum level.
Is there a limit to the amount of EVs I can give my Pokémon?
Fortunately (or perhaps, unfortunately) there are two limits to the amount of EVs a Pokémon can gain.
Each Pokémon can have a maximum of 12,000 EVs.
One stat can have a maximum of 2,000 EVs.
This means you can max every stat to 2,000 to gain the full 12,000 EV's