EV (Effort Values) are values that add up in addition to a Pokemon's stats. Every Pokemon can have up to 510 EVs.
Every 4 EVs contributes one point to the stat. There will however be a minimum of two unused EV points, since 510 isn't perfectly divisible by 4.
Each Pokemon can have up to 252 EVs in one stat, adding a total of 63 additional points. (In Generations 3-5 however they could have up to 255 EVs, although it doesn't really do anything and is really a waste of extra EVs. Pokemon transferred with 255 EVs from those generations however will have those EVs reduced to 252.
There are multiple ways to get EVs.
Different Pokemon species have EV yields - for example, defeating a Bulbasaur will give you 1 Special Attack EV.
You can also get EVs through Vitamin items (HP Up, Protein, etc.). However, before Generation 8, Vitamins couldn't raise an EV stat above 100.
You can also get EVs through Feather items. It will give one EV point to the Pokemon. There isn't a limit unlike the Vitamins.
You can also make your Pokemon hold a Power item (for example, Power Weight). Whenever it gains experience, it will gain 4 EV values depending on what Power item was held. It however halves the speed of the holder.
In Generation 6, there's a thing called Super Training where you can partake in games that'll give you punching bags, one for each stat. There are some additional punching bags as well that can help with your progress. When a Pokemon uses a punching bag, they gain a certain amount of EVs.
Hopefully this helped.
