EVs are basically what you get by battling certain Pokemon. They determine the stats you get in what stat when you level up.
Ex: My Oshawott battles a Lillipup. Lillipup gives one Attack EV, so I get one if I defeat Lillipup. Four EVs give you one more point in that stat, so if I battle four Lillipups and lvel up, I'll get one attack point.
Natures are just stat boosters, but they work differently. They are determined randomly*. Each nature increases one stat and decreases another.
Ex: Adamant increases attack and decreases special attack.
Some natures increase and decrease the same stat, such as Hardy. therefore, there is no change in stats.
*If a Pokemon with synchronize is at the front of your party, and you find a wild Poekmon, that Pokemon is mroe likely to have the same nature as the Synchronize Pokemon.