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 unconscious and -10 means that a character is dead. A character on 0 or below may be bleeding out or otherwise slowly dying.

In 4e, 0 HP means a character is unconscious and dying. Until he is stabilized by an ally, he must make a DC10 Saving Throw each turn in order to remain stable. A character dies after three failed Saving Throws between extended rests. A character also dies should he or she reach an HP total is that equal to or less than half their regular HP value. A common cause of death in 4e is ongoing damage or being damaged from area attacks while dying.