Ability

Almost everything a Player Character does is affected by his abilities: Strength (STR), Dexterity (DEX), Constitution (CON), Intelligence (INT), Wisdom (WIS), and Charisma (CHA).

Abilities are also sometimes named attributes or stats.

Ability modifiers
Each ability has a modifier that is 0 for an ability score of 10 and increases by 1 every 2 ability points above (or decreases by 1 every 2 ability points below). This modifier is the number you apply to the die roll when your character tries to do something related to that ability. You also use the modifier with some numbers that aren’t die rolls. A positive modifier is called a bonus, and a negative modifier is called a penalty.

When an ability modifier changes, all attributes associated with that modifier change accordingly. Note that a character does not retroactively get additional skill points for previous levels if she increases her intelligence; however, a character who increases her constitution will get additional hit points.

See the modifier page for a table giving the modifier tied to an ability score.