Charisma measures a character's force of personality, persuasiveness, ability to lead, and physical attractiveness.
Some of the things which a character's Charisma score affects are:
Class Abilities- A Paladin or Cleric's ability to turn undead is determined by their Charisma score. In addition, Paladins, Bards and Spirit Shamans have other abilities which use their Charisma modifier.
Spellcasting- Many classes use Charisma to cast spells. Bards, Sorcerers and Favored Souls must have a Charisma score of 10 + the spell's level to cast a spell of a given level. For instance, a Favored Soul must have a Charisma score of 15 in order to cast a 5th level spell. In addition, Bards, Sorcerers, Warlocks and Spirit Shamans use their Charisma modifier to determine the Difficulty Class (DC) of their spells.
Charisma is most important for Clerics, Warlocks, Paladins, Sorcerers, Favored Souls and Spirit Shamans. All characters benefit from having a high Charisma when speaking with others in the world since it modifies the player's Diplomacy, Bluff, and Intimidate skill checks.