HP

HP stands for Hit Points. They determine a character's staying power against obstacles which harm you, and when you hit zero the character is rendered useless under most circumstances. Hit points are notoriously abstract in that they don't have a direct representation in the real world - that is, there is no IRL system of categorizing wounds equivalent to varying amounts of HP damage.

In D&D 3.5 0 HP means that a character is disabled, and can only do half of what they could do (either move, or a normal action, but not both). At -1 to -9 they are unconscious and dying, losing 1 hp per turn until they are stabilized -- either by an ally's using magical healing, or succeeding on a Heal skill check, or the player rolling a 1 on d10 each turn. Once stabilized, they are still unconscious but can wake up by rolling 1 on d10 each turn. A conscious character with negative hp is disabled as if they had 0 hp, but doesn't regain hp up to 0. If a character's hp reaches -10, they are immediately dead. Taking 50hp of damage from a single attack forces a DC15 fortitude check; failure means death from massive damage.

In 4e, 0 HP means a character is unconscious and has 'dying' status, and must be stabilized before they die. Until an ally uses their turn to stabilize them, the dying character must make a Saving Throw (roll d20, hoping to get 11-20) each turn to stabilize themselves. A character who has dying status for three turns is dead, even if they aren't three turns in a row; taking an eight hour "extended" rest resets the count ("three strikes yer out"). Dying character will still take damage from attacks, environment or area effects, and will be immediately dead if they take half their maximum hitpoints after reaching 0 hp, even if they still have dying saving throws left or they were stabilized. (ie. a character with 24 hit points is unconscious and dying at 0, and immediately dead at -12 hit points.) A common cause of death in 4e is ongoing damage that continues after they fall to 0hp, or area attacks while dying.