Basically IVs cannot be changed, but EVs can. IVs are already determined when hatched. You can count IVs as bonus points. They range from 0-31. They are basically added to your stat once you reach Lv. 100. For example my Latios has 0 IVs in Sp Attack. At Lv. 100 with EV Training, it's special attack is 350 lets say. Compare that to one with 31 IVs in Sp Attack, which will have 381 as it's Sp Attack stat with the same Evs.