Ev's are effort values given to you by beating certain pokemon.
Each pokemon can only have a total of 510 evs
Only 255 can be put into one stat (but you only want 252)
4 evs = 1 stat up
These evs will naturally raise a pokemon's stat to make it stronger than average in that area, nature helps with it to.
For example, our friend Golbat, gives 2 speed evs. Beat two of them, and you get 4, 1 stat up in speed.
With pokerus you will get 4 just by beating one of them. So you beat two and get 8, 2 stat ups in speed
With macho brace, its already being doubled. So its 8 for beating two, and then an extra 8 more because pokerus doubles the final amount. So thats 4 stat ups just by beating up two of them, this will make lots of things go by faster no?
For pokemon like Gallade, unless your using him as an anti-lead, you generally going to want him as a choice bander/physical sweeper, so they should go into attack and speed.
Smogon provides strategies for certain pokemon, tiers, and articles are always constantly updated to be kept up to date with the ever changing metagame. Serebii provides a more indepth explanation on evs here;
http://www.serebii.net/pokedex-rs/ev.shtml
And smogon is located here, for strategies, and guides for certain things and pokemon;
http://www.smogon.com/
All these things are commonly used in competitive play in pokemon
This stuff is only for the really competitive players, and it makes pokemon super complex, so playing casually is just a way to make it simpler
