Both IVs (Individual Values) and EVs (Effort Values) are hidden stats those affect to stats of pokemon.
IVs are randomly default index formed immediately when you encounter a wild pokemon and they will go with that pokemon as ever (can never change). IVs make the individual pokemon to be nearby unique in his/her species, while there almost are no other individuals have the same IV-set to him/her. That's why they called "Individual Values". When pokemon reach to 100th level, each 1 IV-point will add 1 point to stats of pokemon.
EVs are index formed when you are training your pokemon, gained when your pokemon beat a wild one. EVs make your pokemon have growing tendency to follow certain stats that you want, make that stats to be higher than natural, make your pokemon stronger than other individuals in his/her species. That's why they called "Effort Values". When pokemon reach to 100th level, each 4 EV-points will add 1 point to stats of pokemon.
For example:
Stats of a lv. 100 Pikachu with high IV-set and full 510 EVs:
Stats of a lv. 100 Pikachu with zero IVs and no EVs:
As you see, the Pikachu with higher IVs/EVs-point have higher stats, and of course, is stronger than other one.
Spc.Atk stats with 31 IV-points and 252 EV-points is 199, while Spc.Atk with 0 IV-points and 0 EV-points in only 105. The range 94 points between 2 Spc.Atk stats: 105 + 31 + 252/4 = 199