When a Druid's animal companion's hit points drops to 0 (or lower), does it die? Or does it becomes disabled (unconscious) and can it be healed by the party cleric? What about Wizard familiars?
All living creatures:
Die at −10 HP, and are Dying from −1 to −9 HP. They can be brought back to life with spells like raise dead. Animal companions and familiars don’t have any particular special rules for this.
Constructs and the undead are just destroyed when they hit 0 HP. They, and Outsiders and Elementals, are much harder to bring back to life.
Can be replaced relatively easily:
Not so easily replaced; the ceremony to get one costs 100 gp, and more importantly,
Note that this XP loss happens even if you bring the familiar back to life with e.g. raise dead.