A philosophical zombie is a hypothetical being that is indistinguishable from a normal human being except in that it lacks conscious experience. When a zombie is poked with a sharp object, for example, it does not feel any pain though it behaves exactly as if it does.
The notion of a philosophical zombie is used mainly in thought experiments intended to support arguments against forms of physicalism such as materialism and functionalism. Since a zombie is physiologically indistinguishable from human beings, its hypothetical possibility is an argument for a consciousness that is more than the sum of neurological pathways and brain states.
Though philosophical zombies are widely used in thought experiments, the detailed articulation of the concept is not always the same. Philosophical zombies were introduced primarily to argue against specific types of physicalism such as behaviorism, according to which mental states exist solely as behavior.
Belief, desire, thought, consciousness, and so on, are simply certain kinds of behavior or tendencies towards behaviors. A philosophical zombie that is behaviorally indistinguishable from a normal human being, but lacks conscious experiences, is therefore not logically possible according to the behaviorist.