>>14457133EVs are Effort Values.
A Pokemom can have 510 total Effort Values assigned to it. Defeating Pokemon assigns effort values along with Exp Points, based on which Pokemon you beat (for example I think Audino gives +2 HP EVs).
Every 4 EVs is equal to a single stat point at level 100 (they scale based on the level stat multiplier); so if you have 252 EVs in Atk your mon would have 63 more points in its Atk stat at level 100 than if you had simply Rare Candy'd it up.
But each stat can only have 255 EVs (which, at level 100, means 252; if you can't get to 256, you may as well save the extra few EVs you get for something else). The idea is you can assign a max of 510 points across 6 stats which can each take about half; this lets you shape the final stats of your Pokemon. Maybe you want to max out speed and attack; maybe you want to spread the stats across defense enough that you can survive a hit from Earthquake but still outspeed your opponent. Etc. it's all strategy.
IVs, or individual values, are like EVs in that they affect your mon's stat totals. But they're randomly assigned to each mon from a value between 0 and 31; you can do some breeding tricks to get the one or two most important ones up near 30-31, but the odds of getting all 31s across the board is slim. Basically they add variety; some Gyarados are just "naturally stronger" than others.
Those two things, plus nature (which is randomly assigned, and boosts one stat by 10% and reduces another by 10%) collectively account for the ways that identical mons can have different stat totals.
In competitive battling, you want a stat total which suits your purposes, so you try to get a good nature and IVs, and assign EVs appropriately, to get your desired stat spread.
