Ivs are stuck the whole time, you can't change them, ev points get added on every time you kill a Pokemon, and after you get 510 ev points total you can't get anymore and you're stuck with what you got.

Iv points can be from 0-31 in each stat, a Pokemon at level 100 with an hp iv of 28 will have a stat 28 points higher than a Pokemon with an hp iv of 0.

A Pokemon can have 255 ev points in each stat, but no more than 510 total. After it reaches that cap, it'll gain no more ev points. Ev points are 1/4 as effective as iv points, so a Pokemon with 255 ev points at level 100 will have a stat 63 points higher than a Pokemon with 0 evs in that stat.

Natures are just as important, they raise one stat by 1.1 and lower one by 0.9.

WHEEEEEERES YOOUUUR HEEAAAAD AAAT