Injury and Death

There are many ways in which a character can face mortal peril. Below are some of the most commonly encountered forms.

Hit point loss

What Hit Points Represent
Hit points mean two things in the game world: the ability to take physical punishment and keep going, and the ability to turn a serious blow into a less serious one. As a character gains hit points, they are not only tougher and able to power through worse punishments, they are also able to better minimize the amount of physical harm that damaging sources actually do to them. These representations are all flavor, and all hit points are counted the same mechanically.
Hit Point Damage
Damage alone does not mechanically affect a character's performance until they reach 0 or lower. While it's encouraged to role play the effects of said damage outside of combat, a 1 hit point character is technically every bit as capable as one at full hit points.
Disabled (0 hit points)
When a character's hit point total is exactly 0, they remain conscious but are Disabled.
A Disabled character is unable to walk or stand unaided (whether by a wall, crutch of some form, or assisting character), and can only take a single Move Action or equivalent each round without suffering further damage. If a Disabled character performs a Standard Action or Full-Round Action (or any other action considered to be strenuous, regardless of the action type), at the end of that action they take 1 damage. If this damage puts them below 0, they are now Dying. A Disabled character can perform a Swift Action, Immediate Action, or Free Action as normal, but are still subject to damage if the action is considered by the DM to be strenuous.
Dying (below 0 hit points)
A character below 0 hit points is Dying. A Dying character is Unconscious and in mortal peril.
Each round on a Dying character's initiative, they roll a DC 10 Constitution check. If the check succeeds, the character's state does not worsen, and they repeat the process the next round. If the check fails, they progress closer to death. A natural 1 counts as two failed checks, and a natural 20 puts the character at 0 hit points and Disabled. If the character accumulates 3 failed checks, the character is Dead. If a character recieves hit points from Fast Healing or Regeneration, they do not have to roll a check that round.
If a dying character recieves direct healing (such as from a cure spell or lay on hands), they are healed as though they were at -1 HP, regardless of their actual negative hit point total before recieving healing. A character at negative hit points that does not have the Dying condition (such as with the Diehard feat) does not gain this benefit.
Instant Death
A character who's hp is reduced low enough, whether by the blow that knocks them Unconscious or further damage, may die outright. If a character's negative hit point total is greater than half their maximum hit points (minimum 10), they must make a DC 20 Fortitude Save or die outright. A character who's negative hit point total exceeds their maximum hit points (minimum 15) dies outright with no save.
Nonlethal Damage
Nonlethal damage, also referred to as temporary or subdual damage, is counted up separately from regular hit point damage. When a creature's nonlethal damage total meets their current hit points, they are Staggered, and when their non-lethal damage total exceeds their current hit points, they are knocked Unconscious. Non-lethal damage heals at a rate of 1 hp per hour. Whenever a creature regains normal hit points, they can also remove an equal amount of non-lethal damage. A creature cannot have more nonlethal damage than their maximum hit point total. Any nonlethal damage in excess of this is converted to lethal damage.

Ability Score Loss

Some attacks reduce the opponent’s score in one or more abilities. This loss can be temporary Ability Damaged or permanent Ability Drained.
While any loss is debilitating, losing all points in an ability score can be devastating.
Strength 0 means that the character cannot move at all. He lies Helpless on the ground.
Dexterity 0 means that the character cannot move at all. He stands motionless, rigid, and Helpless.
Constitution 0 means that the character is Dead.
Intelligence 0 means that the character cannot think and is Unconscious in a coma-like stupor, Helpless.
Wisdom 0 means that the character is withdrawn into a deep sleep filled with nightmares, Helpless.
Charisma 0 means that the character is withdrawn into a catatonic, coma-like stupor, Helpless.
Ability Damage is recovered at a rate of 1 point per day to each damaged ability score (or double if the character gets complete bed rest). Ability Drain does not recover naturally. Both Ability Damage and Ability Drain can be recovered through the use of some spells, such as lesser restoration and restoration.

Level Drain

As described in Level Drain, if a character is drained an amount of levels equal to or greater than their level (or Hit Dice if applicable), then they are immediately slain.