Undead are once-living creatures animated by spiritual or supernatural forces.

An undead creature has the following features.

d8 Hit Die.
Base attack bonus equal to 3/4 total Hit Dice (medium progression).
Good Will Saves.
Skill points equal to 4 + Int modifier (minimum 1) per Hit Die. Many undead, however, are mindless and gain no skill points or feats. The following are class skills for undead: Climb, Disguise, Fly, Intimidate, Knowledge (arcana), Knowledge (religion), Perception, Sense Motive, Spellcraft, and Stealth.
Traits: An undead creature possesses the following traits (unless otherwise noted in a creature's entry).

No Constitution score. Undead use their Charisma score in place of their Constitution score when calculating hit points, Fortitude saves, and any special ability that relies on Constitution(such as when calculating a breath weapon’s DC).
Darkvision 60 feet.
Immunity to all mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms).
Immunity to bleed, death effects, disease, paralysis, poison, sleep effects, and stunning.
Not subject to nonlethal damage, ability drain, or energy drain. Immune to damage to its physical ability scores (Constitution, Dexterity, and Strength), as well as to exhaustion and fatigue effects.
Cannot heal damage on its own if it has no Intelligence score, although it can be healed. Negative energy (such as an inflict spell) can heal undead creatures. The fast healing special quality works regardless of the creature's Intelligence score.
Immunity to any effect that requires a Fortitude save (unless the effect also works on objects or is harmless).
Not at risk of death from massive damage, but is immediately destroyed when reduced to 0 hit points.
Not affected by raise dead and reincarnate spells or abilities. Resurrection and true resurrection can affect undead creatures. These spells turn undead creatures back into the living creatures they were before becoming undead.
Proficient with its natural weapons, all simple weapons, and any weapons mentioned in its entry.
Proficient with whatever type of armor (light, medium, or heavy) it is described as wearing, as well as all lighter types. Undead not indicated as wearing armor are not proficient with armor. Undead are proficient with shields if they are proficient with any form of armor.
Undead do not breathe, eat, or sleep.
Per the spell magic jar: "Undead creatures are powered by negative energy. Only sentient undead creatures have, or are, souls." This is an important sentence when considering any spells or effects which reference "souls."

A construct has the following features.

d10 Hit Die.
Base attack bonus equal to total Hit Dice (fast progression).
No good saving throws.
Skill points equal to 2 + Int modifier (minimum 1) per Hit Die. However, most constructs are mindless and gain no skill points or feats. Constructs do not have any class skills, regardless of their Intelligence scores.
Traits: A construct possesses the following traits (unless otherwise noted in a creature's entry).

No Constitution score. Any DCs or other Statistics that rely on a Constitution score treat a construct as having a score of 10 (no bonus or penalty).
Low-light vision.
Darkvision 60 feet.
Immunity to all mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms).
Immunity to bleed, disease, death effects, necromancy effects, paralysis, poison, sleep effects, and stunning.
Cannot heal damage on its own, but often can be repaired via exposure to a certain kind of effect (see the creature's description for details) or through the use of the Craft Construct feat. Constructs can also be healed through spells such as make whole. A construct with the fast healing special quality still benefits from that quality.
Not subject to ability damage, ability drain, fatigue, exhaustion, energy drain, or nonlethal damage.
Immunity to any effect that requires a Fortitude save (unless the effect also works on objects, or is harmless).
Not at risk of death from massive damage. Immediately destroyed when reduced to 0 hit points or less.
A construct cannot be raised or resurrected.
A construct is hard to destroy, and gains bonus hit points based on size, as shown on the following table.
Proficient with its natural weapons only, unless generally humanoid in form, in which case proficient with any weapon mentioned in its entry.
Proficient with no armor.
Constructs do not breathe, eat, or sleep.