When your attack succeeds, you deal damage. The type of weapon used determines the amount of damage you deal. Effects that modify weapon damage apply to unarmed strikes and the natural physical attack forms of creatures.
Damage reduces a target’s current hit points.
If penalties reduce the damage result to less than 1, a hit still deals 1 point of damage. The only exception is if the target has damage reduction.
When you hit with a melee or thrown weapon, including a sling, add your Strength modifier to the damage result. A Strength penalty or a bonus - the latter only if you have the feat Bow Strength - applies on attacks made with any bow. Certain weapons use different ability modifier on damage rolls (list).
When you deal damage with a weapon in your off hand, you add only ½ your Strength bonus. Note that monks get their full Strength bonus to off-hand unarmed attacks.
Wielding a Weapon Two-Handed
When you deal damage with a weapon that you are wielding two-handed, you add 1½ times your Strength bonus.
Sometimes you multiply damage by some factor, such as on a critical hit. Roll the damage (with all modifiers) multiple times and total the results. Note: When you multiply damage more than once, each multiplier works off the original, unmultiplied damage.
Certain creatures and magical effects can cause temporary ability damage (a reduction to an ability score).
A few classes and monsters have access to unique sources of damage which is not listed above. Examples include a dark monk's death touch, a ranger's bonus against favored enemies, a fighter's kensei, and a rogue's sneak attack.
Damage by Color
Orange numbers show normal damage. Purple numbers show extra damage. Yellow numbers show reduced damage. Green numbers indicate healing. (Some types of attacks can heal enemies.)