Each ability partially describes your character and affects some of his or her actions.
Strength measures your character's muscle and physical power. This ability is especially important for fighters, barbarians, paladins, rangers, and monks because it helps them prevail in combat. Strength also limits the amount of equipment your character can carry.
You apply your character's Strength modifier to:
Melee attack rolls.
Damage rolls when using a melee weapon or a thrown weapon (including a sling). (Exceptions: Off-hand attacks receive only one-half the character's Strength bonus, while two-handed attacks receive one and a half times the Strength bonus. A Strength penalty, but not a bonus, applies to attacks made with a bow that is not a composite bow.)
Strength checks (for breaking down doors and the like).
Dexterity measures hand-eye coordination, agility, reflexes, and balance. This ability is the most important one for rogues, but it's also high on the list for characters who typically wear light or medium armor (rangers and barbarians) or no armor at all (monks, wizards, and sorcerers), and for anyone who wants to be a skilled archer.
You apply your character's Dexterity modifier to:
Ranged attack rolls, including those for attacks made with bows, crossbows, throwing axes, and other ranged weapons.
Armor Class (AC), provided that the character can react to the attack.
Reflex saving throws, for avoiding fireballs and other attacks that you can escape by moving quickly.
Constitution represents your character's health and stamina. A Constitution bonus increases a character's hit points, so the ability is important for all classes.
You apply your character's Constitution modifier to:
Each roll of a Hit Die (though a penalty can never drop a result below 1-that is, a character always gains at least 1 hit point each time he or she advances in level).
Fortitude saving throws, for resisting poison and similar threats.
If a character's Constitution score changes enough to alter his or her Constitution modifier, the character's hit points also increase or decrease accordingly.
Intelligence determines how well your character learns and reasons. This ability is important for wizards because it affects how many spells they can cast, how hard their spells are to resist, and how powerful their spells can be. It's also important for any character who wants to have a wide assortment of skills.
You apply your character's Intelligence modifier to:
The number of languages your character knows at the start of the game.
The number of skill points gained each level. (But your character always gets at least 1 skill point per level.)
A wizard gains bonus spells based on her Intelligence score. The minimum Intelligence score needed to cast a wizard spell is 10 + the spell's level.
An animal has an Intelligence score of 1 or 2. A creature of humanlike intelligence has a score of at least 3.
Wisdom describes a character's willpower, common sense, perception, and intuition. While Intelligence represents one's ability to analyze information, Wisdom represents being in tune with and aware of one's surroundings. Wisdom is the most important ability for clerics and druids, and it is also important for paladins and rangers. If you want your character to have acute senses, put a high score in Wisdom. Every creature has a Wisdom score.
You apply your character's Wisdom modifier to:
Clerics, druids, paladins, and rangers get bonus spells based on their Wisdom scores. The minimum Wisdom score needed to cast a cleric, druid, paladin, or ranger spell is 10 + the spell's level.
Charisma measures a character's force of personality, persuasiveness, personal magnetism, ability to lead, and physical attractiveness. This ability represents actual strength of personality, not merely how one is perceived by others in a social setting. Charisma is most important for paladins, sorcerers, and bards. It is also important for clerics, since it affects their ability to turn undead. Every creature has a Charisma score.
You apply your character's Charisma modifier to:
Checks that represent attempts to influence others.
Turning checks for clerics and paladins attempting to turn zombies, vampires, and other undead.
Sorcerers and bards get bonus spells based on their Charisma scores. The minimum Charisma score needed to cast a sorcerer or bard spell is 10 + the spell's level.
When an ability score changes, all attributes associated with that score change accordingly. A character does not retroactively get additional skill points for previous levels if she increases her intelligence.