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Intelligence (Int) describes how well a character learns and reasons.

Intelligence is the key ability for Arcana, History, and Religion skill checks.

In addition, a character's Intelligence modifier or Dexterity modifier, whichever is higher, will contribute to Reflex defense. If a character wears light armor, his or her Intelligence modifier or Dexterity modifier, whichever is higher, also contributes to Armor Class.

Intelligence and Dexterity are redundant with each other for the purpose of defense bonuses, but Dexterity has the additional benefit of improving initiative checks. For this reason, Intelligence can be a dump stat for characters who don't depend on Intelligence for attack rolls, class features, or skill checks.

Classes with Intelligence as a primary key ability
Classes with Intelligence as a secondary key ability
avenger (Wis primary, Censure of Retribution or Censure of Unity)
bard (Cha primary, Virtue of Cunning)
invoker (Wis primary, Covenant of Preservation)
shaman (Wis primary, Elemental or Stalker Spirit)
warlock (Con and/or Cha primary, original warlock, Star Pact hexblade, or Star Pact binder)
warlord (Str primary, Resourceful, Skirmishing, or Tactical Presence)
Races with a bonus to Intelligence
genasi (Int/Str)
eladrin, elf, gnome, shadar-kai (Int/Dex)
genasi, githyanki, hobgoblin, warforged (Int/Con)
deva, githzerai, hamadryad, shardmind (Int/Wis)
changeling, eladrin, gnome, kalashtar, kenku, pixie, shade, shardmind, tiefling (Int/Cha)