One of the hidden stats in Pokémon is called EV. What are these? What do they do? Is it something I need to worry about at all? Does this exist in every Pokémon from the beginning? If not, in what generation did it start?
EVs stand for Effort Values. When you catch a wild Pokemon, it has no EVs, and they go up through battle. Every time it participates in fainting another Pokemon (either in the wild or a trainer battle), a Pokemon will be given specific EVs depending on the Pokemon it defeated (for example, Machop will give Attack EVs, whereas Abra will give Special Attack EVs; you can look up which EVs a Pokemon gives on any reasonable online Pokedex). Pokemon holding an Exp. Share will also gain EVs even if they don't participate in the battle.
EVs are all tied to one of the basic stats (Attack, Defense, Special Attack, Special Defense, Speed, and HP). Your Pokemon can have a total of 255 EVs in a single stat, and 510 EVs overall.
For each 4 EVs you have in a given stat, that stat will be increased by 1 at level 100. This is scaled down to whatever level your Pokemon currently is, so for example at level 50, you need 8 EVs in a given stat to increase that stat by 1.
Once you have 510 total EVs, additional EVs that Pokemon would gain are simply lost. Because of this, if you are really set on having a min/maxed Pokemon, you need to keep track of its EVs and do very specific training on Pokemon that will raise the correct stats and not raise the wrong stats until the EV training is complete. Otherwise, you are liable to "waste" EVs in stats you don't care about.
EVs have existed since the beginning, but they were overhauled in the 3rd generation. I am not as well-versed in the specifics from before that change, although you can read about them at Bulbapedia.
There is a Pokemon disease called "Pokerus" (which was introduced in Generation 2) that will double the rate of EV gain. Since EV training is very, very tedious, this rare disease is highly coveted, and it is recommended to always keep an actively infected Pokemon in your box so that you have access to the disease whenever convenient.