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Charisma (Cha) measures a character's force of personality, persuasiveness, and leadership.

Charisma is the key ability for Bluff, Diplomacy, Intimidate, and Streetwise skill checks.

In addition, a character's Charisma modifier or Wisdom modifier, whichever is higher, will contribute to Will defense.

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

Classes with Charisma as a primary key ability
paladin ("chaladin")
warlock (Dark, Elemental, or Fey Pact original warlock, or hexblade, or binder)
Classes with Charisma as a secondary key ability
barbarian (Str primary, Thaneborn Triumph original barbarian)
battlemind (Con primary)
cleric (Wis primary, templar or sun domain warpriest)
psion (Int primary)
rogue (Dex primary, Artful Dodger scoundrel or thief)
warlord (Str primary, Bravura, Insightful, Inspiring, or Resourceful Presence)
wizard (Int primary, Orb of Deception arcanist)
Races with a bonus to Charisma
dragonborn, vryloka (Cha/Str)
dragonborn, half-elf, hobgoblin, kobold, satyr, tiefling (Cha/Con)
changeling, drow, goblin, halfling, hengeyokai, kenku, pixie, revenant, satyr, shade, vryloka (Cha/Dex)
changeling, eladrin, gnome, kalashtar, kenku, pixie, shade, shardmind, tiefling (Cha/Int)
deva, hamadryad, kalashtar (Cha/Wis)