Constructs are either animated objects or any type of artificially constructed creatures. Most constructs are non-intelligent automotons with neither a soul nor a purpose beyond the service of their creator.
Because few constructs are truly alive, usually operated only by a "spark" of life, most constructs are immune to attacks that specifically target the living. These "dead" constructs possess low-light vision and darkvision and are immunity to mind-altering, poison, sleep, paralysis, stunning, disease-related, and death effects. Neither do these constructs require sleep or take part in any metabolic activities, such as eating or breathing.
The major exception to this rule are living constructs, which include warforged, living mechanical or elemental beasts typically created and built specifically for war.
While Zombies are often referred to as the living dead, they should actually be considered living constructs.