Simply put Ivs are like DNA for pokemon, that add to their stats
The difference between Effort Values and IVs is that Effort Values change over time (by battling and with certain items), whereas IVs will never change. When you encounter a wild Pokemon in battle or an egg is received from the Daycare, the game assigns a number from 0 to 31 to each of its stats (HP, Attack, Defense, Speed, Special Attack and Special Defense) and this number, from now on, will not change. IVs of 31 yield higher stats, while IVs of 0 yield lower stats.
IVs aren't new to the Advance Generations - they've been a big influence in Gold, Silver and Crystal, and while they were present in Red Blue and Yellow, they didn't do much. In GSC, DVs controlled a Pokemon's gender as well as whether it was shiny. In RSE and FRLG, these two characteristics are controlled by a different, hidden value. Apart from affecting stats, IVs also affect the type and the base damage of Hidden Power
Characteristics
The secondary personality trait that Pokemon have in the fourth generation, informally known as "Characteristics," tells you your Pokemon's highest Individual Value (IV). If your Pokemon has two IVs tied for the highest value, the characteristic tells you a random stat. Here's a list of each characteristic and their potential IVs:
HIT POINTS
Characteristic Possible IVs
Often dozes off 1 6 11 16 21 26 31
Loves to eat 0 5 10 15 20 25 30
Likes to relax 4 9 14 19 24 29
Scatters things often 3 8 13 18 23 28
Often scatters things 2 7 12 17 22 27
ATTACK
Characteristic Possible IVs
Likes to thrash about 1 6 11 16 21 26 31
Proud of its power 0 5 10 15 20 25 30
Quick tempered 4 9 14 19 24 29
Likes to fight 3 8 13 18 23 28
A little quick tempered 2 7 12 17 22 27
DEFENSE
Characteristic Possible IVs
Capable of taking hits 1 6 11 16 21 26 31
Sturdy body 0 5 10 15 20 25 30
Good perseverance 4 9 14 19 24 29
Good endurance 3 8 13 18 23 28
Highly persistent 2 7 12 17 22 27
SPECIAL ATTACK
Characteristic Possible IVs
Mischievous 1 6 11 16 21 26 31
Highly curious 0 5 10 15 20 25 30
Very finicky 4 9 14 19 24 29
Often lost in thought 3 8 13 18 23 28
Thoroughly cunning 2 7 12 17 22 27
SPECIAL DEFENSE
Characteristic Possible IVs
Somewhat vain 1 6 11 16 21 26 31
Strong willed 0 5 10 15 20 25 30
Somewhat stubborn 4 9 14 19 24 29
Hates to lose 3 8 13 18 23 28
Strongly defiant 2 7 12 17 22 27
SPEED
Characteristic Possible IVs
Alert to sounds 1 6 11 16 21 26 31
Likes to run 0 5 10 15 20 25 30
Quick to flee 4 9 14 19 24 29
Somewhat of a clown 3 8 13 18 23 28
Impetuous and silly 2 7 12 17 22 27
ORIGINALLY MADE BY THE GOOD PPL OF PSYPOKES