Okay- here we go.
This is EVs: Effort Values, or EVs for short, can have a huge impact on your Pokémon and the battles they participate in. EVs will let Swampert survive Hidden Power Grass, or will let Skarmory outspeed Magneton, and many other things. Having a proper EV spread (distribution of EV points on a Pokémon) can mean the difference between a win and a loss.
A total of 255 EV points can be put onto any one stat, and a total of 510 points total can be given to any Pokémon. However, it is important to note that a stat will only increase after every four EV points. In other words, putting 12 EVs and putting 15 EVs both result in the same thing: a 3 point increase to the stat.
Ugh, okay didja get all that?? IVs: Like EVs, Individual Values also have a large effect on the stats of your Pokémon. However, unlike EVs, you cannot change the IVs of your Pokémon. For each stat, the IV itself is a number from 0, which gives you the lowest possible stat, to 31, which gives you the highest. For example, if you take two Vaporeon (at level 100), one with an ATK IV of 31, and another with an ATK IV of 0, you will see that the difference between the stats is exactly 31. At half the level, the difference becomes half of 31, and so on.
Stats aren't the only thing affected by IVs. The type and Base Power of Hidden Power is also determined by the Pokémon's IVs. In general, the higher the IVs, the stronger Hidden Power will be (with a maximum Base Power of 70). However to control the type, you must specific combinations of IVs. For example, a Pokémon with an IV of 31 in every stat will have a dark-type Hidden Power, and if that Pokémon instead had a Special Attack IV of 0, the Hidden Power becomes type electric, and has a Base Power of 59.